diff --git a/src/diffViewController.ts b/src/diffViewController.ts
index 2a7f2fd..32dac9a 100644
--- a/src/diffViewController.ts
+++ b/src/diffViewController.ts
@@ -145,7 +145,13 @@ export class DiffViewController implements vscode.Disposable {
   pin(document: vscode.TextDocument): void {
     if (!this.isDiffable(document)) return;
     const key = this.uriKey(document);
-    if (this.modes.get(key) !== "snapshot") {
+    const mode = this.modes.get(key);
+    if (mode === undefined) {
+      // never established — the document is not being coedited (#71 item 1)
+      void vscode.window.showWarningMessage("Run ✦ Coedit this Document with Claude first.");
+      return;
+    }
+    if (mode !== "snapshot") {
       void vscode.window.showWarningMessage(
         "Cowriting: this document is git-tracked — commit to advance the baseline.",
       );