diff --git a/src/type/__tests__/introspection-test.ts b/src/type/__tests__/introspection-test.ts
index 2e5f261e946..cb5fe8fa570 100644
--- a/src/type/__tests__/introspection-test.ts
+++ b/src/type/__tests__/introspection-test.ts
@@ -1571,7 +1571,7 @@ describe('Introspection', () => {
     }
 
     // istanbul ignore next (Called only to fail test)
-    function typeResolver(_1, _2, info) {
+    function typeResolver(_1, _2, info): undefined {
       expect.fail(`Called on ${info.parentType.name}::${info.fieldName}`);
     }