diff --git a/packages/frontend-app-api/src/tree/resolveAppNodeSpecs.test.ts b/packages/frontend-app-api/src/tree/resolveAppNodeSpecs.test.ts
index 0be99c4969..b881d8e0f1 100644
--- a/packages/frontend-app-api/src/tree/resolveAppNodeSpecs.test.ts
+++ b/packages/frontend-app-api/src/tree/resolveAppNodeSpecs.test.ts
@@ -44,7 +44,7 @@ function makeExtDef(
 ) {
   return {
     $$type: '@backstage/ExtensionDefinition',
-    T: null as any,
+    T: undefined as any,
     version: 'v1',
     name,
     attachTo: { id: attachId, input: 'default' },
diff --git a/packages/frontend-plugin-api/src/wiring/createExtension.ts b/packages/frontend-plugin-api/src/wiring/createExtension.ts
index 8838f7bf9a..445bbd8a68 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtension.ts
+++ b/packages/frontend-plugin-api/src/wiring/createExtension.ts
@@ -390,7 +390,7 @@ export function createExtension<
   return {
     $$type: '@backstage/ExtensionDefinition',
     version: 'v2',
-    T: null as unknown as T,
+    T: undefined as unknown as T,
     kind: options.kind,
     namespace: options.namespace,
     name: options.name,
diff --git a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts
index cc874f9d03..09c7b9e9a9 100644
--- a/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts
+++ b/packages/frontend-plugin-api/src/wiring/resolveExtensionDefinition.test.ts
@@ -23,7 +23,7 @@ import {
 describe('resolveExtensionDefinition', () => {
   const baseDef = {
     $$type: '@backstage/ExtensionDefinition',
-    T: null as any,
+    T: undefined as any,
     version: 'v2',
     attachTo: { id: '', input: '' },
     disabled: false,
@@ -65,7 +65,7 @@ describe('resolveExtensionDefinition', () => {
 describe('old resolveExtensionDefinition', () => {
   const baseDef = {
     $$type: '@backstage/ExtensionDefinition',
-    T: null as any,
+    T: undefined as any,
     version: 'v1',
     attachTo: { id: '', input: '' },
     disabled: false,