diff --git a/src/p_slopes.c b/src/p_slopes.c
index f8a2752c2f63e2653c9c6c5ec2b2bbcfc8a3ea04..6393ca4b558514e0616dc0423f60d899e29342c9 100644
--- a/src/p_slopes.c
+++ b/src/p_slopes.c
@@ -90,7 +90,7 @@ static void P_ReconfigureVertexSlope(pslope_t *slope)
 
 		// Get angles
 		slope->xydirection = R_PointToAngle2(0, 0, slope->d.x, slope->d.y)+ANGLE_180;
-		slope->zangle = InvAngle(R_PointToAngle2(0, 0, FRACUNIT, slope->zdelta))-1;
+		slope->zangle = InvAngle(R_PointToAngle2(0, 0, FRACUNIT, slope->zdelta));
 	}
 }