diff --git a/polymer/eduke32/build/src/polymer.c b/polymer/eduke32/build/src/polymer.c
index ac6d6743f..0293c3567 100644
--- a/polymer/eduke32/build/src/polymer.c
+++ b/polymer/eduke32/build/src/polymer.c
@@ -2202,6 +2202,8 @@ static void         polymer_updatewall(int16_t wallnum)
     // it also works, bitches
     wal = &wall[wallnum];
     nwallnum = wal->nextwall;
+    if (sectorofwall(wallnum) == -1)
+        return; // yay, corrupt map
     sec = &sector[sectorofwall(wallnum)];
     if (sec->wallptr > wallnum)
         return; // the map is horribly corrupt