diff --git a/lib/AsyncGenerator.php b/lib/AsyncGenerator.php
index 1086373..2285b53 100644
--- a/lib/AsyncGenerator.php
+++ b/lib/AsyncGenerator.php
@@ -71,6 +71,8 @@ final class AsyncGenerator implements Stream
      * @psalm-param TSend $value
      *
      * @return Promise<array>
+     *
+     * @psalm-return Promise<list<TValue>>
      */
     public function send($value): Promise
     {
@@ -83,6 +85,8 @@ final class AsyncGenerator implements Stream
      * @param \Throwable $exception Exception to throw into the async generator.
      *
      * @return Promise<array>
+     *
+     * @psalm-return Promise<list<TValue>>
      */
     public function throw(\Throwable $exception): Promise
     {
diff --git a/lib/Internal/Yielder.php b/lib/Internal/Yielder.php
index ab0ffa0..273099f 100644
--- a/lib/Internal/Yielder.php
+++ b/lib/Internal/Yielder.php
@@ -39,9 +39,6 @@ trait Yielder
     /** @var Deferred[] */
     private $waiting = [];
 
-    /** @var int */
-    private $nextKey = 0;
-
     /** @var int */
     private $consumePosition = 0;
 
@@ -58,9 +55,9 @@ trait Yielder
     private $used = false;
 
     /**
-     * @return Promise<array|null>
+     * @return Promise<array>
      *
-     * @psalm-return Promise<list<TValue>|null>
+     * @psalm-return Promise<list<TValue>>
      */
     public function continue(): Promise
     {
@@ -72,9 +69,9 @@ trait Yielder
      *
      * @psalm-param TSend $value
      *
-     * @return Promise<array|null>
+     * @return Promise<array>
      *
-     * @psalm-return Promise<list<TValue>|null>
+     * @psalm-return Promise<list<TValue>>
      */
     public function send($value): Promise
     {
@@ -88,9 +85,9 @@ trait Yielder
     /**
      * @param \Throwable $exception
      *
-     * @return Promise<array|null>
+     * @return Promise<array>
      *
-     * @psalm-return Promise<list<TValue>|null>
+     * @psalm-return Promise<list<TValue>>
      */
     public function throw(\Throwable $exception): Promise
     {
@@ -104,11 +101,11 @@ trait Yielder
     /**
      * @param Promise<mixed> $promise
      *
-     * @psalm-param Promise<TSend|null>
+     * @psalm-param Promise<TSend|null> $promise
      *
-     * @return Promise<array|null>
+     * @return Promise<array>
      *
-     * @psalm-return Promise<list<TValue>|null>
+     * @psalm-return Promise<list<TValue>>
      */
     private function next(Promise $promise): Promise
     {
@@ -123,11 +120,11 @@ trait Yielder
             $this->sendValues[$position - 1] = $promise;
         }
 
-        if (isset($this->yieldedValues[$position])) {
-            $tuple = $this->yieldedValues[$position];
+        if (\array_key_exists($position, $this->yieldedValues)) {
+            $value = $this->yieldedValues[$position];
             unset($this->yieldedValues[$position]);
 
-            return new Success($tuple);
+            return new Success([$value]);
         }
 
         if ($this->result) {
@@ -190,11 +187,11 @@ trait Yielder
      *
      * @psalm-param TValue $value
      *
-     * @return Promise<TSend> Resolves with the key of the yielded value once the value has been consumed. Fails with
-     *                        the failure reason if the {@see fail()} is called, or with {@see DisposedException} if the
-     *                        stream is destroyed.
+     * @return Promise<mixed> Resolves with the sent value once the value has been consumed. Fails with the failure
+     *                        reason if the {@see fail()} is called, or with {@see DisposedException} if the stream
+     *                        is destroyed.
      *
-     * @psalm-return Promise<TSend>
+     * @psalm-return Promise<TSend|null>
      *
      * @throws \Error If the stream has completed.
      */
@@ -212,14 +209,12 @@ trait Yielder
             throw new \TypeError("Streams cannot yield promises");
         }
 
-        $key = $this->nextKey++;
-        $tuple = [$value, $key];
         $position = $this->yieldPosition++;
 
         if (isset($this->waiting[$position])) {
             $deferred = $this->waiting[$position];
             unset($this->waiting[$position]);
-            $deferred->resolve($tuple);
+            $deferred->resolve([$value]);
 
             // Send-values are indexed as $this->consumePosition - 1, so use $position for the next value.
             if (isset($this->sendValues[$position])) {
@@ -228,7 +223,7 @@ trait Yielder
                 return $promise;
             }
         } else {
-            $this->yieldedValues[$position] = $tuple;
+            $this->yieldedValues[$position] = $value;
         }
 
         $this->backPressure[$position] = $deferred = new Deferred;
diff --git a/lib/Stream.php b/lib/Stream.php
index f36f807..e2f5f25 100644
--- a/lib/Stream.php
+++ b/lib/Stream.php
@@ -10,12 +10,13 @@ namespace Amp;
 interface Stream
 {
     /**
-     * Succeeds with a tuple of the yielded value and key or null if the stream has completed. If the stream fails,
-     * the returned promise will fail with the same exception.
+     * Succeeds with a single element array containing the yielded value if the stream has yielded a value. If the
+     * stream completes the promise resolves with null. If the stream fails, the returned promise will fail with the
+     * same exception.
      *
-     * @return Promise<array|null>
+     * @return Promise<array> Resolves with null if the stream has completed.
      *
-     * @psalm-return Promise<list<TValue>|null>
+     * @psalm-return Promise<list<TValue>>
      *
      * @throws \Throwable The exception used to fail the stream.
      */
diff --git a/lib/functions.php b/lib/functions.php
index 6168f5b..d41d99b 100644
--- a/lib/functions.php
+++ b/lib/functions.php
@@ -811,10 +811,21 @@ namespace Amp\Iterator
         });
     }
 
+    /**
+     * @template TValue
+     *
+     * @param Stream $stream
+     *
+     * @psalm-param Stream<TValue> $stream
+     *
+     * @return Iterator
+     *
+     * @psalm-return Iterator<TValue>
+     */
     function fromStream(Stream $stream): Iterator
     {
         return new Producer(function (callable $emit) use ($stream): \Generator {
-            while (null !== $value = $stream->continue()) {
+            while (null !== $value = yield $stream->continue()) {
                 yield $emit($value);
             }
         });
@@ -837,11 +848,17 @@ namespace Amp\Stream
      * Creates a stream from the given iterable, emitting the each value. The iterable may contain promises. If any
      * promise fails, the returned stream will fail with the same reason.
      *
+     * @template TValue
+     *
      * @param array|\Traversable $iterable Elements to yield.
      * @param int                $delay Delay between elements yielded in milliseconds.
      *
+     * @psalm-param iterable<TValue> $iterable
+     *
      * @return Stream
      *
+     * @psalm-return Stream<TValue>
+     *
      * @throws \TypeError If the argument is not an array or instance of \Traversable.
      */
     function fromIterable(/* iterable */
@@ -873,7 +890,7 @@ namespace Amp\Stream
      * @template TReturn
      *
      * @param Stream $stream
-     * @param callable (TValue $value, int $key): TReturn $onYield
+     * @param callable(TValue $value):TReturn $onYield
      *
      * @psalm-param Stream<TValue> $stream
      *
@@ -884,8 +901,8 @@ namespace Amp\Stream
     function map(Stream $stream, callable $onYield): Stream
     {
         return new AsyncGenerator(static function (callable $yield) use ($stream, $onYield) {
-            while (list($value, $key) = yield $stream->continue()) {
-                yield $yield($onYield($value, $key));
+            while (list($value) = yield $stream->continue()) {
+                yield $yield($onYield($value));
             }
         });
     }
@@ -894,7 +911,7 @@ namespace Amp\Stream
      * @template TValue
      *
      * @param Stream $stream
-     * @param callable(TValue $value, int $key):bool $filter
+     * @param callable(TValue $value):bool $filter
      *
      * @psalm-param Stream<TValue> $stream
      *
@@ -905,9 +922,9 @@ namespace Amp\Stream
     function filter(Stream $stream, callable $filter): Stream
     {
         return new AsyncGenerator(static function (callable $yield) use ($stream, $filter) {
-            while (list($value, $key) = yield $stream->continue()) {
-                if ($filter($value, $key)) {
-                    yield $yield($value, $key);
+            while (list($value) = yield $stream->continue()) {
+                if ($filter($value)) {
+                    yield $yield($value);
                 }
             }
         });
@@ -1027,8 +1044,6 @@ namespace Amp\Stream
      * @psalm-param Stream<TValue> $stream
      *
      * @return Promise<int>
-     *
-     * @psalm-return Promise<int>
      */
     function discard(Stream $stream): Promise
     {
@@ -1059,7 +1074,7 @@ namespace Amp\Stream
     function toArray(Stream $stream): Promise
     {
         return call(static function () use ($stream): \Generator {
-            /** @psalm-var list $array */
+            /** @psalm-var list<TValue> $array */
             $array = [];
 
             while (list($value) = yield $stream->continue()) {
@@ -1070,6 +1085,19 @@ namespace Amp\Stream
         });
     }
 
+    /**
+     * Converts an instance of the deprecated {@see Iterator} into an instance of {@see Stream}.
+     *
+     * @template TValue
+     *
+     * @param Iterator $iterator
+     *
+     * @psalm-param Iterator<TValue> $iterator
+     *
+     * @return Stream
+     *
+     * @psalm-return Stream<TValue>
+     */
     function fromIterator(Iterator $iterator): Stream
     {
         return new AsyncGenerator(function (callable $yield) use ($iterator): \Generator {
diff --git a/test/AsyncGeneratorTest.php b/test/AsyncGeneratorTest.php
index eef33cc..4edf04b 100644
--- a/test/AsyncGeneratorTest.php
+++ b/test/AsyncGeneratorTest.php
@@ -30,7 +30,7 @@ class AsyncGeneratorTest extends BaseTest
                 yield $yield($value);
             });
 
-            $this->assertSame([$value, 0], yield $generator->continue());
+            $this->assertSame([$value], yield $generator->continue());
         });
     }
 
@@ -43,7 +43,7 @@ class AsyncGeneratorTest extends BaseTest
                 $result = yield $yield($value);
             });
 
-            $this->assertSame([$value, 0], yield $generator->continue());
+            $this->assertSame([$value], yield $generator->continue());
             $this->assertNull(yield $generator->send($send));
             $this->assertSame($result, $send);
         });
@@ -62,7 +62,7 @@ class AsyncGeneratorTest extends BaseTest
             $promise1 = $generator->continue();
             $promise2 = $generator->send($send);
 
-            $this->assertSame([$value, 0], yield $promise1);
+            $this->assertSame([$value], yield $promise1);
             $this->assertNull(yield $promise2);
             $this->assertSame($result, $send);
         });
@@ -84,7 +84,7 @@ class AsyncGeneratorTest extends BaseTest
             $promise1 = $generator->continue();
             $promise2 = $generator->throw($exception);
 
-            $this->assertSame([$value, 0], yield $promise1);
+            $this->assertSame([$value], yield $promise1);
             $this->assertNull(yield $promise2);
             $this->assertSame($result, $exception);
         });
@@ -104,7 +104,7 @@ class AsyncGeneratorTest extends BaseTest
                 }
             });
 
-            $this->assertSame([$value, 0], yield $generator->continue());
+            $this->assertSame([$value], yield $generator->continue());
             $this->assertNull(yield $generator->throw($exception));
             $this->assertSame($result, $exception);
         });
@@ -147,7 +147,7 @@ class AsyncGeneratorTest extends BaseTest
                 return $value;
             });
 
-            $this->assertSame([null, 0], yield $generator->continue());
+            $this->assertSame([null], yield $generator->continue());
             $this->assertNull(yield $generator->continue());
             $this->assertSame($value, yield $generator->getReturn());
         });
diff --git a/test/Stream/FilterTest.php b/test/Stream/FilterTest.php
index dd6dfbd..0bc58e6 100644
--- a/test/Stream/FilterTest.php
+++ b/test/Stream/FilterTest.php
@@ -3,12 +3,12 @@
 namespace Amp\Test\Stream;
 
 use Amp\AsyncGenerator;
+use Amp\PHPUnit\AsyncTestCase;
 use Amp\PHPUnit\TestException;
 use Amp\Stream;
 use Amp\StreamSource;
-use Amp\Test\BaseTest;
 
-class FilterTest extends BaseTest
+class FilterTest extends AsyncTestCase
 {
     public function testNoValuesEmitted()
     {
diff --git a/test/YielderTraitTest.php b/test/YielderTraitTest.php
index 4aa23c5..ac9eb00 100644
--- a/test/YielderTraitTest.php
+++ b/test/YielderTraitTest.php
@@ -34,7 +34,7 @@ class YielderTraitTest extends TestCase
             $promise = $this->source->yield($value);
             $stream = $this->source->createStream();
 
-            $this->assertSame([$value, 0], yield $stream->continue());
+            $this->assertSame([$value], yield $stream->continue());
 
             $this->assertInstanceOf(Promise::class, $promise);
             $this->assertNull(yield $promise);
@@ -60,7 +60,7 @@ class YielderTraitTest extends TestCase
     {
         Loop::run(function () {
             $this->source->yield(null);
-            $this->assertSame([null, 0], yield $this->source->createStream()->continue());
+            $this->assertSame([null], yield $this->source->createStream()->continue());
         });
     }
 
@@ -115,7 +115,7 @@ class YielderTraitTest extends TestCase
 
             $this->assertNull(yield $this->source->yield($value));
 
-            $this->assertSame([$value, 0], yield $promise);
+            $this->assertSame([$value], yield $promise);
         });
     }