diff --git a/src/viewport.cpp b/src/viewport.cpp
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -2786,7 +2786,7 @@ calc_heightdiff_single_direction:;
 					}
 				}
 
-				if (t0 != 1 || t1 != 1) {
+				if (dx != 1 || dy != 1) {
 					int heightdiff = CalcHeightdiff(style, 0, t0, t1);
 
 					params[index++] = dx;