diff --git a/developer/src/kmc-kmn/src/kmw-compiler/javascript-strings.ts b/developer/src/kmc-kmn/src/kmw-compiler/javascript-strings.ts
index 0b356a9a18c..55eca7791fe 100644
--- a/developer/src/kmc-kmn/src/kmw-compiler/javascript-strings.ts
+++ b/developer/src/kmc-kmn/src/kmw-compiler/javascript-strings.ts
@@ -885,7 +885,7 @@ export function JavaScript_OutputString(fk: KMX.KEYBOARD, FTabStops: string, fkp
         len = -1;
         break;
       default:
-        callbacks.reportMessage(KmwCompilerMessages.Error_NotSupportedInKeymanWebOutput({line: fkp.Line, code: GetCodeName(rec.Code)}));
+        callbacks.reportMessage(KmwCompilerMessages.Error_NotSupportedInKeymanWebOutput({line: fkp?.Line ?? 0, code: GetCodeName(rec.Code)}));
         Result += '';
       }
     }
diff --git a/developer/src/kmc-kmn/test/fixtures/kmw/error_not_supported_in_keyman_web_output.kmn b/developer/src/kmc-kmn/test/fixtures/kmw/error_not_supported_in_keyman_web_output.kmn
new file mode 100644
index 00000000000..f8d20f49565
--- /dev/null
+++ b/developer/src/kmc-kmn/test/fixtures/kmw/error_not_supported_in_keyman_web_output.kmn
@@ -0,0 +1,10 @@
+store(&VERSION) '9.0'
+store(&NAME) 'error_not_supported_in_keyman_web_output'
+store(&TARGETS) 'web'
+
+begin Unicode > use(main)
+
+group(main) using keys
+
+c `return` is supported only in KMX; see also #11545
+nomatch > return
diff --git a/developer/src/kmc-kmn/test/kmw/test-kmw-messages.ts b/developer/src/kmc-kmn/test/kmw/test-kmw-messages.ts
index 28c12e87044..4ab741a4cc9 100644
--- a/developer/src/kmc-kmn/test/kmw/test-kmw-messages.ts
+++ b/developer/src/kmc-kmn/test/kmw/test-kmw-messages.ts
@@ -71,4 +71,11 @@ describe('KmwCompilerMessages', function () {
   //   await testForMessage(this, ['kmw', '....kmn'], KmwCompilerMessages.ERROR_TouchLayoutFileDoesNotExist);
   // });
 
+  // ERROR_NotSupportedInKeymanWebOutput
+
+  it('should generate ERROR_NotSupportedInKeymanWebOutput if the command is not supported in output for KeymanWeb', async function() {
+    await testForMessage(this, ['kmw', 'error_not_supported_in_keyman_web_output.kmn'], KmwCompilerMessages.ERROR_NotSupportedInKeymanWebOutput);
+  });
+
+
 });