diff --git a/src/__tests__/utils/pt9-import-error.test.ts b/src/__tests__/utils/pt9-import-error.test.ts
index deebc0c9..90bc3b04 100644
--- a/src/__tests__/utils/pt9-import-error.test.ts
+++ b/src/__tests__/utils/pt9-import-error.test.ts
@@ -28,4 +28,12 @@ describe('isPt9TooLargeError', () => {
     expect(isPt9TooLargeError(new Error('boom'))).toBe(false);
     expect(isPt9TooLargeError(MARKER_MESSAGE)).toBe(false);
   });
+
+  it('rejects a platform error carrying no message instead of throwing', () => {
+    // Narrows as a platform error despite omitting the message the type declares as required.
+    const noMessage: unknown = { platformErrorVersion: 1 };
+
+    expect(() => isPt9TooLargeError(noMessage)).not.toThrow();
+    expect(isPt9TooLargeError(noMessage)).toBe(false);
+  });
 });
diff --git a/src/utils/pt9-import-error.ts b/src/utils/pt9-import-error.ts
index fd21d31e..1b96cf86 100644
--- a/src/utils/pt9-import-error.ts
+++ b/src/utils/pt9-import-error.ts
@@ -13,6 +13,11 @@ const PT9_TOO_LARGE_MARKER = 'PT9 interlinear data is too large';
  */
 export function isPt9TooLargeError(error: unknown): boolean {
   if (isPlatformError(error))
-    return error.code === 'RESOURCE_EXHAUSTED' || error.message.includes(PT9_TOO_LARGE_MARKER);
+    // The narrowing proves less than the type does: a value can satisfy it without carrying the
+    // declared message, and we want to answer rather than throw.
+    return (
+      error.code === 'RESOURCE_EXHAUSTED' ||
+      (typeof error.message === 'string' && error.message.includes(PT9_TOO_LARGE_MARKER))
+    );
   return error instanceof Error && error.message.includes(PT9_TOO_LARGE_MARKER);
 }