diff --git a/polymer/eduke32/build/src/engine.c b/polymer/eduke32/build/src/engine.c
index c4dbda62c..6ab960b18 100644
--- a/polymer/eduke32/build/src/engine.c
+++ b/polymer/eduke32/build/src/engine.c
@@ -2751,7 +2751,7 @@ static WSHELPER_DECL void calc_bufplc(intptr_t *bufplc, int32_t lw, int32_t tsiz
 //    if (i >= tsizx)
     {
         if (tsizx < 0)
-            i = (uint32_t)i % tsizx;
+            i = (uint32_t)i % -tsizx;
         else
             i &= tsizx;
     }
@@ -2761,6 +2761,8 @@ static WSHELPER_DECL void calc_bufplc(intptr_t *bufplc, int32_t lw, int32_t tsiz
     else
         i <<= tsizy;
 
+//    Bassert(i >= 0 && i < tilesizx[globalpicnum]*tilesizy[globalpicnum]);
+
     // Address is at the first row of tile storage (which is column-major).
     *bufplc = waloff[globalpicnum] + i;
 }