diff --git a/src/r_plane.c b/src/r_plane.c
index c54b32382..194f85c4a 100644
--- a/src/r_plane.c
+++ b/src/r_plane.c
@@ -705,9 +705,9 @@ void R_CalculateSlopeVectors(pslope_t *slope, fixed_t planeviewx, fixed_t planev
 	n.z = -xscale * cos(ang);
 
 	ang = ANG2RAD(planeangle);
-	temp = P_GetSlopeZAt(slope, planeviewx + yscale * FLOAT_TO_FIXED(sin(ang)), planeviewy + yscale * FLOAT_TO_FIXED(cos(ang)));
+	temp = P_GetSlopeZAt(slope, planeviewx + FLOAT_TO_FIXED(yscale * sin(ang)), planeviewy + FLOAT_TO_FIXED(yscale * cos(ang)));
 	m.y = FIXED_TO_FLOAT(temp) - zeroheight;
-	temp = P_GetSlopeZAt(slope, planeviewx + xscale * FLOAT_TO_FIXED(cos(ang)), planeviewy - xscale * FLOAT_TO_FIXED(sin(ang)));
+	temp = P_GetSlopeZAt(slope, planeviewx + FLOAT_TO_FIXED(xscale * cos(ang)), planeviewy - FLOAT_TO_FIXED(xscale * sin(ang)));
 	n.y = FIXED_TO_FLOAT(temp) - zeroheight;
 
 	if (ds_powersoftwo)