diff --git a/src/pathfinder/npf/npf.cpp b/src/pathfinder/npf/npf.cpp
--- a/src/pathfinder/npf/npf.cpp
+++ b/src/pathfinder/npf/npf.cpp
@@ -245,8 +245,8 @@ static uint NPFSlopeCost(AyStarNode *cur
 	/* Get the height on both sides of the tile edge.
 	 * Avoid testing the height on the tile-center. This will fail for halftile-foundations.
 	 */
-	int z1 = GetSlopePixelZ(x1 + dx4, y1 + dy4);
-	int z2 = GetSlopePixelZ(x2 - dx4, y2 - dy4);
+	int z1 = GetSlopePixelZ(x1 + dx4, y1 + dy4, true);
+	int z2 = GetSlopePixelZ(x2 - dx4, y2 - dy4, true);
 
 	if (z2 - z1 > 1) {
 		/* Slope up */