diff --git a/Form/DataTransformer/HashToKeyValueArrayTransformer.php b/Form/DataTransformer/HashToKeyValueArrayTransformer.php
index e27894d..1c91f08 100644
--- a/Form/DataTransformer/HashToKeyValueArrayTransformer.php
+++ b/Form/DataTransformer/HashToKeyValueArrayTransformer.php
@@ -41,7 +41,7 @@ public function reverseTransform($value)
                 throw new TransformationFailedException;
             }
 
-            if (array_key_exists($data['key'], $return)) {
+            if (isset($data['key'], $return[$data['key']])) {
                 throw new TransformationFailedException('Duplicate key detected');
             }
 
diff --git a/composer.json b/composer.json
index 17f15b3..bb38807 100644
--- a/composer.json
+++ b/composer.json
@@ -7,7 +7,7 @@
         "psr-4": { "Burgov\\Bundle\\KeyValueFormBundle\\": "" }
     },
     "require": {
-        "php": ">=5.3.3",
+        "php": ">=5.3.3|^7.4",
         "symfony/form": "^2.3|^3.0|^4.0|^5.0"
     },
     "conflict": {