diff --git a/src/Psl/Arr/count_values.php b/src/Psl/Arr/count_values.php
index 67123d3..fc99488 100644
--- a/src/Psl/Arr/count_values.php
+++ b/src/Psl/Arr/count_values.php
@@ -27,6 +27,7 @@ function count_values(iterable $values): array
     $result = [];
 
     foreach ($values as $value) {
+        Psl\invariant(is_arraykey($value), 'Expected all values to be of type array-key, value of type (%s) provided.', gettype($value));
         /** @psalm-var int $count */
         $count = idx($result, $value, 0);
         $result[$value] = $count + 1;
diff --git a/src/Psl/Arr/flip.php b/src/Psl/Arr/flip.php
index bb712e7..ddf1a62 100644
--- a/src/Psl/Arr/flip.php
+++ b/src/Psl/Arr/flip.php
@@ -26,6 +26,7 @@ function flip(iterable $iterable): array
 {
     $result = [];
     foreach ($iterable as $k => $v) {
+        Psl\invariant(is_arraykey($v), 'Expected all values to be of type array-key, value of type (%s) provided.', gettype($v));
         $result[$v] = $k;
     }
 
diff --git a/src/Psl/Arr/is_arraykey.php b/src/Psl/Arr/is_arraykey.php
new file mode 100644
index 0000000..70311ab
--- /dev/null
+++ b/src/Psl/Arr/is_arraykey.php
@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psl\Arr;
+
+use Psl\Str;
+
+/**
+ * Finds whether a variable is an array key.
+ *
+ * @psalm-param mixed   $key
+ *
+ * @psalm-assert-if-true array-key $key
+ */
+function is_arraykey($key): bool
+{
+    return Str\is_string($key) || is_int($key);
+}
diff --git a/tests/Psl/Arr/FlipTest.php b/tests/Psl/Arr/FlipTest.php
index 04880eb..e2e6166 100644
--- a/tests/Psl/Arr/FlipTest.php
+++ b/tests/Psl/Arr/FlipTest.php
@@ -32,12 +32,4 @@ class FlipTest extends TestCase
             ],
         ];
     }
-
-    public function testFlipThrowsForNonArrayKeyValues(): void
-    {
-        $this->expectException(Exception\InvariantViolationException::class);
-        $this->expectExceptionMessage('Expected all values to be of type array-key, value of type (resource) provided.');
-
-        Arr\flip([STDOUT]);
-    }
 }