diff --git a/src/type/definition.ts b/src/type/definition.ts
index ed658a8fc7..abf5a3a202 100644
--- a/src/type/definition.ts
+++ b/src/type/definition.ts
@@ -753,7 +753,7 @@ export interface GraphQLObjectTypeExtensions<_TSource = any, _TContext = any> {
 export class GraphQLObjectType<TSource = any, TContext = any> {
   name: string;
   description: Maybe<string>;
-  isTypeOf: Maybe<GraphQLIsTypeOfFn<TSource, TContext>>;
+  isTypeOf: Maybe<GraphQLIsTypeOfFn<unknown, TContext>>;
   extensions: Readonly<GraphQLObjectTypeExtensions<TSource, TContext>>;
   astNode: Maybe<ObjectTypeDefinitionNode>;
   extensionASTNodes: ReadonlyArray<ObjectTypeExtensionNode>;
@@ -932,7 +932,7 @@ export interface GraphQLObjectTypeConfig<TSource, TContext> {
   description?: Maybe<string>;
   interfaces?: ThunkReadonlyArray<GraphQLInterfaceType>;
   fields: ThunkObjMap<GraphQLFieldConfig<TSource, TContext>>;
-  isTypeOf?: Maybe<GraphQLIsTypeOfFn<TSource, TContext>>;
+  isTypeOf?: Maybe<GraphQLIsTypeOfFn<unknown, TContext>>;
   extensions?: Maybe<Readonly<GraphQLObjectTypeExtensions<TSource, TContext>>>;
   astNode?: Maybe<ObjectTypeDefinitionNode>;
   extensionASTNodes?: Maybe<ReadonlyArray<ObjectTypeExtensionNode>>;