Index: viewport.c
===================================================================
--- viewport.c	(revision 6250)
+++ viewport.c	(working copy)
@@ -297,6 +297,7 @@
 {
 	Point pt;
 	int a,b;
+	uint maxx, maxy;
 
 	if ( (uint)(x -= vp->left) >= (uint)vp->width ||
 				(uint)(y -= vp->top) >= (uint)vp->height) {
@@ -315,7 +316,19 @@
 	b = x-y;
 #endif
 
-	if ((uint)a < MapMaxX() * TILE_SIZE && (uint)b < MapMaxY() * TILE_SIZE) {
+	maxx = MapMaxX() * TILE_SIZE;
+	maxy = MapMaxY() * TILE_SIZE;
+
+	/* we need to move variables in to the valid range if they happen to be outside as
+	GetTileZoomCenterWindow() can call here with invalid x and/or y if the user tries
+	to zoom out along the sides of the map */
+	if ((uint)a >= maxx) {
+		if (a < 0) a = 0; else a = ((int)maxx) - 1;
+	}
+	if ((uint)b >= maxy) {
+		if (b < 0) b = 0; else b = ((int)maxy) - 1;
+	}
+
 		uint z;
 
 		z = GetSlopeZ(a,     b    ) / 2;
@@ -326,9 +339,6 @@
 
 		pt.x = a + z;
 		pt.y = b + z;
-	} else {
-		pt.x = pt.y = -1;
-	}
 
 	return pt;
 }