diff --git a/composer.json b/composer.json
index 228872a2b1d..c1416a74654 100644
--- a/composer.json
+++ b/composer.json
@@ -17,6 +17,7 @@
         "doctrine/inflector": "^1.0",
         "psr/cache": "^1.0",
         "psr/container": "^1.0",
+        "symfony/config": "^2.0 || ^3.0 || ^4.0",
         "symfony/http-foundation": "^3.4 || ^4.0",
         "symfony/http-kernel": "^3.4 || ^4.0",
         "symfony/property-access": "^3.4 || ^4.0",
diff --git a/src/Metadata/Resource/Factory/OperationResourceMetadataFactory.php b/src/Metadata/Resource/Factory/OperationResourceMetadataFactory.php
index 4db5f5045d5..6ddb0cce174 100644
--- a/src/Metadata/Resource/Factory/OperationResourceMetadataFactory.php
+++ b/src/Metadata/Resource/Factory/OperationResourceMetadataFactory.php
@@ -14,6 +14,7 @@
 namespace ApiPlatform\Core\Metadata\Resource\Factory;
 
 use ApiPlatform\Core\Metadata\Resource\ResourceMetadata;
+use Symfony\Component\Config\Exception\LoaderLoadException;
 
 /**
  * Creates or completes operations.
@@ -123,6 +124,13 @@ private function normalize(bool $collection, ResourceMetadata $resourceMetadata,
                 $supported ? $operation['method'] = $upperOperationName : $operation['route_name'] = $operationName;
             }
 
+            if (strtoupper($operation['method']) !== $operation['method']) {
+                $message = sprintf('The method "%s" of operation "%s" should be in uppercase.', $operation['method'], $operationName);
+
+                // See DelegatingLoader::load to understand why we use a LoaderLoadException
+                throw new LoaderLoadException($resourceMetadata->getShortName(), 'api-platform', null, new \InvalidArgumentException($message));
+            }
+
             $newOperations[$operationName] = $operation;
         }