diff --git a/src/iter.php b/src/iter.php
index fe4a945..a11201d 100644
--- a/src/iter.php
+++ b/src/iter.php
@@ -208,7 +208,7 @@ function filter(callable $predicate, $iterable) {
  *      => 120
  *
  * @param callable $function Reduction function:
- *                           mixed function(mixed $acc, mixed $value)
+ *                           mixed function(mixed $acc, mixed $value, mixed $key)
  * @param array|Traversable $iterable Iterable to reduce
  * @param mixed $startValue Start value for accumulator.
  *                          Usually identity value of $function.
@@ -219,8 +219,8 @@ function reduce(callable $function, $iterable, $startValue = null) {
     _assertIterable($iterable, 'Second argument');
 
     $acc = $startValue;
-    foreach ($iterable as $value) {
-        $acc = $function($acc, $value);
+    foreach ($iterable as $key => $value) {
+        $acc = $function($acc, $value, $key);
     }
     return $acc;
 }
@@ -242,7 +242,7 @@ function reduce(callable $function, $iterable, $startValue = null) {
  *      => iter(1, 2, 6, 24, 120)
  *
  * @param callable $function Reduction function:
- *                           mixed function(mixed $acc, mixed $value)
+ *                           mixed function(mixed $acc, mixed $value, mixed $key)
  * @param array|Traversable $iterable   Iterable to reduce
  * @param mixed $startValue Start value for accumulator.
  *                          Usually identity value of $function.
@@ -253,8 +253,8 @@ function reductions(callable $function, $iterable, $startValue = null) {
     _assertIterable($iterable, 'Second argument');
 
     $acc = $startValue;
-    foreach ($iterable as $value) {
-        $acc = $function($acc, $value);
+    foreach ($iterable as $key => $value) {
+        $acc = $function($acc, $value, $key);
         yield $acc;
     }
 }
diff --git a/test/iterTest.php b/test/iterTest.php
index 036413e..c1f931a 100644
--- a/test/iterTest.php
+++ b/test/iterTest.php
@@ -134,9 +134,30 @@ public function testReduce() {
         $this->assertSame(120, reduce(fn\operator('*'), range(1, 5), 1));
     }
 
+    public function testComplexReduce() {
+        $this->assertSame('abcdef', reduce(function ($acc, $value, $key) {
+            return $acc . $key . $value;
+        }, ['a' => 'b', 'c' => 'd', 'e' => 'f'], ''));
+    }
+
     public function testReductions() {
-        $this->assertSame([1, 3, 6, 10, 15], toArrayWithKeys(reductions(fn\operator('+'), range(1, 5), 0)));
-        $this->assertSame([1, 2, 6, 24, 120], toArrayWithKeys(reductions(fn\operator('*'), range(1, 5), 1)));
+        $this->assertSame(
+            [1, 3, 6, 10, 15],
+            toArrayWithKeys(reductions(fn\operator('+'), range(1, 5), 0))
+        );
+        $this->assertSame(
+            [1, 2, 6, 24, 120],
+            toArrayWithKeys(reductions(fn\operator('*'), range(1, 5), 1))
+        );
+    }
+
+    public function testComplexReductions() {
+        $this->assertSame(
+            ['ab', 'abcd', 'abcdef'],
+            toArrayWithKeys(reductions(function ($acc, $value, $key) {
+                return $acc . $key . $value;
+            }, ['a' => 'b', 'c' => 'd', 'e' => 'f'], ''))
+        );
     }
 
     public function testAnyAll() {