diff --git a/src/include/ui.h b/src/include/ui.h
index aec4b74a0..0f9541b0a 100644
--- a/src/include/ui.h
+++ b/src/include/ui.h
@@ -317,7 +317,7 @@ extern int MapTileGetViewport(int, int);
     /// FIXME: Short one line docu
 extern void CycleViewportMode(int);
     /// FIXME: Short one line docu
-extern void SetViewportMode(void);
+extern void SetViewportMode(ViewportMode mode);
 
 #endif /* SPLIT_SCREEN_SUPPORT */