diff --git a/lib/Listener.php b/lib/Listener.php
index 1973cc3..95a3180 100644
--- a/lib/Listener.php
+++ b/lib/Listener.php
@@ -22,13 +22,13 @@ class Listener {
     private $values = [];
 
     /** @var \Amp\Deferred[] */
-    private $deferreds = [];
+    private $backPressure = [];
 
     /** @var int */
     private $position = -1;
 
     /** @var \Amp\Deferred|null */
-    private $deferred;
+    private $waiting;
 
     /** @var bool */
     private $resolved = false;
@@ -45,19 +45,19 @@ class Listener {
     public function __construct(Stream $stream) {
         $this->stream = $stream;
 
-        $deferred = &$this->deferred;
+        $waiting = &$this->waiting;
         $values = &$this->values;
-        $deferreds = &$this->deferreds;
+        $backPressure = &$this->backPressure;
         $resolved = &$this->resolved;
 
-        $this->stream->listen(static function ($value) use (&$deferred, &$values, &$deferreds, &$resolved) {
+        $this->stream->listen(static function ($value) use (&$waiting, &$values, &$backPressure, &$resolved) {
             $values[] = $value;
-            $deferreds[] = $pressure = new Deferred;
+            $backPressure[] = $pressure = new Deferred;
 
-            if ($deferred !== null) {
-                $temp = $deferred;
-                $deferred = null;
-                $temp->resolve(true);
+            if ($waiting !== null) {
+                $deferred = $waiting;
+                $waiting = null;
+                $deferred->resolve(true);
             }
 
             if ($resolved) {
@@ -70,21 +70,21 @@ class Listener {
         $result = &$this->result;
         $error = &$this->exception;
 
-        $this->stream->when(static function ($exception, $value) use (&$deferred, &$result, &$error, &$resolved) {
+        $this->stream->when(static function ($exception, $value) use (&$waiting, &$result, &$error, &$resolved) {
             $resolved = true;
 
             if ($exception) {
                 $result = null;
                 $error = $exception;
-                if ($deferred !== null) {
-                    $deferred->fail($exception);
+                if ($waiting !== null) {
+                    $waiting->fail($exception);
                 }
                 return;
             }
 
             $result = $value;
-            if ($deferred !== null) {
-                $deferred->resolve(false);
+            if ($waiting !== null) {
+                $waiting->resolve(false);
             }
         });
     }
@@ -95,7 +95,7 @@ class Listener {
     public function __destruct() {
         $this->resolved = true;
 
-        foreach ($this->deferreds as $deferred) {
+        foreach ($this->backPressure as $deferred) {
             $deferred->resolve();
         }
     }
@@ -112,11 +112,17 @@ class Listener {
      * resolved. If the stream fails, the returned promise will fail with the same exception.
      *
      * @return \AsyncInterop\Promise<bool>
+     *
+     * @throws \Error If the prior promise returned from this method has not resolved.
      */
     public function advance(): Promise {
-        if (isset($this->deferreds[$this->position])) {
-            $future = $this->deferreds[$this->position];
-            unset($this->values[$this->position], $this->deferreds[$this->position]);
+        if ($this->waiting !== null) {
+            throw new \Error("The prior promise returned must resolve before invoking this method again");
+        }
+
+        if (isset($this->backPressure[$this->position])) {
+            $future = $this->backPressure[$this->position];
+            unset($this->values[$this->position], $this->backPressure[$this->position]);
             $future->resolve();
         }
 
@@ -136,8 +142,8 @@ class Listener {
             return new Success(false);
         }
 
-        $this->deferred = new Deferred;
-        return $this->deferred->promise();
+        $this->waiting = new Deferred;
+        return $this->waiting->promise();
     }
 
     /**
@@ -196,8 +202,8 @@ class Listener {
         $this->values = [];
         $this->position = -1;
 
-        $deferreds = $this->deferreds;
-        $this->deferreds = [];
+        $deferreds = $this->backPressure;
+        $this->backPressure = [];
         foreach ($deferreds as $deferred) {
             $deferred->resolve();
         }
diff --git a/test/ObserverTest.php b/test/ListenerTest.php
similarity index 93%
rename from test/ObserverTest.php
rename to test/ListenerTest.php
index be1458e..39ae5f8 100644
--- a/test/ObserverTest.php
+++ b/test/ListenerTest.php
@@ -180,4 +180,15 @@ class ListenerTest extends \PHPUnit_Framework_TestCase {
             $listener->getResult();
         }));
     }
+
+    /**
+     * @expectedException \Error
+     * @expectedExceptionMessage The prior promise returned must resolve before invoking this method again
+     */
+    public function testConsecutiveAdvanceCalls() {
+        $emitter = new Emitter;
+        $listener = new Listener($emitter->stream());
+        $listener->advance();
+        $listener->advance();
+    }
 }