diff --git a/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts b/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts
index 63273dd9abacf595ec54bd61189c1e73b58b0adc..7801fcbf337926067c07add32291edd72a2f1e1d 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts
@@ -230,7 +230,7 @@ class MainThreadDocumentAndEditorStateComputer {
 
 		// editor: only take those that have a not too large model
 		const editors = new Map<string, TextEditorSnapshot>();
-		let activeEditor: string | undefined = undefined;
+		let activeEditor: string | null = null; // Strict null work. This doesn't like being undefined!
 
 		for (const editor of this._codeEditorService.listCodeEditors()) {
 			if (editor.isSimpleWidget) {