diff --git a/src/Mapping/Driver/SymfonyFileLocator.php b/src/Mapping/Driver/SymfonyFileLocator.php
index 09339fae..553c5150 100644
--- a/src/Mapping/Driver/SymfonyFileLocator.php
+++ b/src/Mapping/Driver/SymfonyFileLocator.php
@@ -11,10 +11,8 @@
 use RuntimeException;
 
 use function array_keys;
-use function assert;
 use function is_dir;
 use function is_file;
-use function is_int;
 use function realpath;
 use function sprintf;
 use function str_replace;
@@ -222,12 +220,12 @@ public function findMappingFile(string $className): string
             }
         }
 
-        $pos = strrpos($className, '\\');
-        assert(is_int($pos));
+        $pos            = strrpos($className, '\\');
+        $shortClassName = $pos === false ? $className : substr($className, $pos + 1);
 
         throw MappingException::mappingFileNotFound(
             $className,
-            substr($className, $pos + 1) . $this->fileExtension,
+            $shortClassName . $this->fileExtension,
         );
     }
 
diff --git a/tests/Mapping/SymfonyFileLocatorTest.php b/tests/Mapping/SymfonyFileLocatorTest.php
index ff7767d8..0b58d5b4 100644
--- a/tests/Mapping/SymfonyFileLocatorTest.php
+++ b/tests/Mapping/SymfonyFileLocatorTest.php
@@ -188,6 +188,15 @@ public function testFindMappingFileNotFound(): void
         $locator->findMappingFile('Foo\\stdClass2');
     }
 
+    public function testFindMappingFileNotFoundForClassInRootNamespace(): void
+    {
+        $locator = new SymfonyFileLocator([], '.yml');
+
+        $this->expectException(MappingException::class);
+        $this->expectExceptionMessage("No mapping file found named 'DateTime.yml' for class 'DateTime'.");
+        $locator->findMappingFile('DateTime');
+    }
+
     public function testFindMappingFileLeastSpecificNamespaceFirst(): void
     {
         // Low -> High