diff --git a/poly2d/p2d_attrib.c b/poly2d/p2d_attrib.c
index 42ece8c..f4edf5d 100644
--- a/poly2d/p2d_attrib.c
+++ b/poly2d/p2d_attrib.c
@@ -12,6 +12,7 @@
 
 
 #include <stdbool.h>
+#include <stdio.h>
 #include <math.h>
 #include <assert.h>
 
@@ -27,7 +28,7 @@ static double angle_3(const struct v2d *a, const struct v2d *b,
     const struct v2d *c)
 {
 	double ax, ay, bx, by;
-	double aa, bb;
+	double cs, aa, bb;
 	double cross, angle;
 
 	ax = b->x - a->x;
@@ -42,8 +43,13 @@ static double angle_3(const struct v2d *a, const struct v2d *b,
 	aa = hypot(ax, ay);
 	bb = hypot(bx, by);
 
-	angle = acos((ax * bx + ay * by) / aa / bb) / M_PI * 180.0;
-
+	cs = (ax * bx + ay * by) / aa / bb;
+	if (cs <= -1)
+		angle = 180;
+	else if (cs >= 1)
+		angle = 0;
+	else
+		angle = acos(cs) / M_PI * 180.0;
 	return cross >= 0 ? angle : -angle;
 }