diff --git a/src/viewport.cpp b/src/viewport.cpp
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -1728,7 +1728,7 @@ void ViewportDoDraw(const ViewPort *vp, 
  */
 static void ViewportDrawChk(const ViewPort *vp, int left, int top, int right, int bottom)
 {
-	if (ScaleByZoom(bottom - top, vp->zoom) * ScaleByZoom(right - left, vp->zoom) > (int)(180000 * ZOOM_LVL_BASE * ZOOM_LVL_BASE)) {
+	if ((int64)ScaleByZoom(bottom - top, vp->zoom) * (int64)ScaleByZoom(right - left, vp->zoom) > (int64)(180000 * ZOOM_LVL_BASE * ZOOM_LVL_BASE)) {
 		if ((bottom - top) > (right - left)) {
 			int t = (top + bottom) >> 1;
 			ViewportDrawChk(vp, left, top, right, t);