diff --git a/src/Type/UnionTypeHelper.php b/src/Type/UnionTypeHelper.php
index 7f8936899e8..e0fed341f1c 100644
--- a/src/Type/UnionTypeHelper.php
+++ b/src/Type/UnionTypeHelper.php
@@ -70,6 +70,10 @@ public static function getConstantStrings(array $types): array
 			$strings[] = $type->getConstantStrings();
 		}
 
+		if ($strings === []) {
+			return [];
+		}
+
 		return array_merge(...$strings);
 	}