diff --git a/src/keyboard.h b/src/keyboard.h
index e44f6a800eb..c397c9b6e2f 100644
--- a/src/keyboard.h
+++ b/src/keyboard.h
@@ -43,7 +43,7 @@ extern int this_command_key_count;
    generate switch-frame events.  This may be cleared by functions
    like Fselect_frame, to make sure that a switch-frame event is
    generated by the next character.  */
-Lisp_Object internal_last_event_frame;
+extern Lisp_Object internal_last_event_frame;
 #endif