diff --git a/src/video/cocoa/SDL_cocoawindow.m b/src/video/cocoa/SDL_cocoawindow.m
index 63a8b7659..ad56e7b94 100644
--- a/src/video/cocoa/SDL_cocoawindow.m
+++ b/src/video/cocoa/SDL_cocoawindow.m
@@ -2228,7 +2228,7 @@ Cocoa_GetWindowDisplayIndex(_THIS, SDL_Window * window)
 
     /* Not recognized via CHECK_WINDOW_MAGIC */
     if (data == nil) {
-        return 0;
+        return SDL_SetError("Window data not set");
     }
 
     /* NSWindow.screen may be nil when the window is off-screen. */