diff --git a/poly2d/p2d_attrib.c b/poly2d/p2d_attrib.c
index 406240b..87a476b 100644
--- a/poly2d/p2d_attrib.c
+++ b/poly2d/p2d_attrib.c
@@ -28,19 +28,23 @@ static double angle_3(const struct v2d *a, const struct v2d *b,
 {
 	double ax, ay, bx, by;
 	double aa, bb;
-	double angle;
+	double cross, angle;
 
 	ax = b->x - a->x;
 	ay = b->y - a->y;
 	bx = c->x - b->x;
 	by = c->y - b->y;
 
+	cross = ax * by - ay * bx;
+	if (!cross)
+		return 0;
+
 	aa = hypot(ax, ay);
 	bb = hypot(bx, by);
 
 	angle = acos((ax * bx + ay * by) / aa / bb) / M_PI * 180.0;
 
-	return ax * by - ay * bx >= 0 ? angle : -angle;
+	return cross >= 0 ? angle : -angle;
 }
 
 /*