diff --git a/src/document/migrate-scene-document.ts b/src/document/migrate-scene-document.ts
index c8e69059..97e9f164 100644
--- a/src/document/migrate-scene-document.ts
+++ b/src/document/migrate-scene-document.ts
@@ -904,7 +904,8 @@ function readPlayerStartNavigationMode(value: unknown, label: string) {
     value,
     label,
     "firstPerson",
-    isPlayerStartNavigationMode
+    (candidate): candidate is "firstPerson" | "thirdPerson" =>
+      typeof candidate === "string" && isPlayerStartNavigationMode(candidate)
   );
 }