diff --git a/src/main_gui.cpp b/src/main_gui.cpp
index 47156f690d..6a5781897e 100644
--- a/src/main_gui.cpp
+++ b/src/main_gui.cpp
@@ -128,7 +128,7 @@ void ShowNetworkGiveMoneyWindow(CompanyID company)
 
 /* Zooms a viewport in a window in or out
  * No button handling or what so ever */
-bool DoZoomInOutWindow(int how, Window *w)
+bool DoZoomInOutWindow(ZoomStateChange how, Window *w)
 {
 	ViewPort *vp;
 
@@ -136,6 +136,7 @@ bool DoZoomInOutWindow(int how, Window *w)
 	vp = w->viewport;
 
 	switch (how) {
+		case ZOOM_NONE: NOT_REACHED();
 		case ZOOM_IN:
 			if (vp->zoom == ZOOM_LVL_MIN) return false;
 			vp->zoom = (ZoomLevel)((int)vp->zoom - 1);
diff --git a/src/viewport_func.h b/src/viewport_func.h
index dbe965d4f2..a2dd0df18a 100644
--- a/src/viewport_func.h
+++ b/src/viewport_func.h
@@ -35,12 +35,12 @@ void UpdateViewportPosition(Window *w);
  */
 void MarkAllViewportsDirty(int left, int top, int right, int bottom);
 
-bool DoZoomInOutWindow(int how, Window *w);
+bool DoZoomInOutWindow(ZoomStateChange how, Window *w);
 void ZoomInOrOutToCursorWindow(bool in, Window * w);
 Point GetTileZoomCenterWindow(bool in, Window * w);
 void HandleZoomMessage(Window *w, const ViewPort *vp, byte widget_zoom_in, byte widget_zoom_out);
 
-static inline void MaxZoomInOut(int how, Window *w)
+static inline void MaxZoomInOut(ZoomStateChange how, Window *w)
 {
 	while (DoZoomInOutWindow(how, w)) {};
 }
diff --git a/src/viewport_type.h b/src/viewport_type.h
index 165d20c20b..8f65f85060 100644
--- a/src/viewport_type.h
+++ b/src/viewport_type.h
@@ -51,7 +51,7 @@ struct ViewportSign {
 	void MarkDirty() const;
 };
 
-enum {
+enum ZoomStateChange {
 	ZOOM_IN   = 0,
 	ZOOM_OUT  = 1,
 	ZOOM_NONE = 2, // hack, used to update the button status