diff --git a/graphql/index.d.ts b/graphql/index.d.ts
index 14b29144eb..3eb0111d25 100644
--- a/graphql/index.d.ts
+++ b/graphql/index.d.ts
@@ -2826,7 +2826,7 @@ declare module "graphql/utilities/TypeInfo" {
             // NOTE: this experimental optional second parameter is only needed in order
             // to support non-spec-compliant codebases. You should never need to use it.
             // It may disappear in the future.
-            getFieldDefFn: getFieldDef
+            getFieldDefFn?: getFieldDef
         );
 
         getType(): GraphQLOutputType;