diff --git a/src/heretic/am_map.c b/src/heretic/am_map.c
index aa56b67d..fa385860 100644
--- a/src/heretic/am_map.c
+++ b/src/heretic/am_map.c
@@ -1259,8 +1259,7 @@ void AM_drawGrid(int color)
     // Figure out start of vertical gridlines
     start = m_x;
     if ((start - bmaporgx) % (MAPBLOCKUNITS << FRACBITS))
-        start += (MAPBLOCKUNITS << FRACBITS)
-            - ((start - bmaporgx) % (MAPBLOCKUNITS << FRACBITS));
+        start -= ((start - bmaporgx) % (MAPBLOCKUNITS << FRACBITS));
     end = m_x + m_w;
 
     // draw vertical gridlines
@@ -1276,8 +1275,7 @@ void AM_drawGrid(int color)
     // Figure out start of horizontal gridlines
     start = m_y;
     if ((start - bmaporgy) % (MAPBLOCKUNITS << FRACBITS))
-        start += (MAPBLOCKUNITS << FRACBITS)
-            - ((start - bmaporgy) % (MAPBLOCKUNITS << FRACBITS));
+        start -= ((start - bmaporgy) % (MAPBLOCKUNITS << FRACBITS));
     end = m_y + m_h;
 
     // draw horizontal gridlines