diff --git a/src/p_slopes.cpp b/src/p_slopes.cpp
index 66b3d07476..7413922e25 100644
--- a/src/p_slopes.cpp
+++ b/src/p_slopes.cpp
@@ -178,8 +178,8 @@ void P_SetSlope (secplane_t *plane, bool setCeil, int xyangi, int zangi, const D
 	if (ib_compatflags & BCOMPATF_SETSLOPEOVERFLOW)
 	{
 		// We have to consider an integer multiplication overflow here.
-		norm[0] = FixedToFloat(FloatToFixed(zang.Cos()) * FloatToFixed(xyang.Cos()));
-		norm[1] = FixedToFloat(FloatToFixed(zang.Cos()) * FloatToFixed(xyang.Sin()));
+		norm[0] = FixedToFloat(FloatToFixed(zang.Cos()) * FloatToFixed(xyang.Cos())) / 65536.;
+		norm[1] = FixedToFloat(FloatToFixed(zang.Cos()) * FloatToFixed(xyang.Sin())) / 65536.;
 	}
 	else
 	{
diff --git a/wadsrc/static/compatibility.txt b/wadsrc/static/compatibility.txt
index d964cbf75f..73a4f5af2f 100644
--- a/wadsrc/static/compatibility.txt
+++ b/wadsrc/static/compatibility.txt
@@ -27,6 +27,7 @@
 	setwallyscale 717 front bot 1.090909
 
 	setslopeoverflow
+	polyobj
 }
 
 B2D8DA03489D1C67F60DC87FBC4EA338 // map01 - Massmouth 2