diff --git a/source/blender/blenlib/intern/math_vector.c b/source/blender/blenlib/intern/math_vector.c
index 7f578fe305d..812e2b3e63d 100644
--- a/source/blender/blenlib/intern/math_vector.c
+++ b/source/blender/blenlib/intern/math_vector.c
@@ -208,6 +208,13 @@ float angle_signed_v2v2(const float v1[2], const float v2[2])
 
 float angle_normalized_v3v3(const float v1[3], const float v2[3])
 {
+	/* double check they are normalized */
+#ifdef DEBUG
+	float test;
+	BLI_assert(fabsf((test = len_squared_v3(v1)) - 1.0f) < 0.0001f || fabsf(test) < 0.0001f);
+	BLI_assert(fabsf((test = len_squared_v3(v2)) - 1.0f) < 0.0001f || fabsf(test) < 0.0001f);
+#endif
+
 	/* this is the same as acos(dot_v3v3(v1, v2)), but more accurate */
 	if (dot_v3v3(v1, v2) < 0.0f) {
 		float vec[3];
@@ -224,6 +231,13 @@ float angle_normalized_v3v3(const float v1[3], const float v2[3])
 
 float angle_normalized_v2v2(const float v1[2], const float v2[2])
 {
+	/* double check they are normalized */
+#ifdef DEBUG
+	float test;
+	BLI_assert(fabsf((test = len_squared_v2(v1)) - 1.0f) < 0.0001f || fabsf(test) < 0.0001f);
+	BLI_assert(fabsf((test = len_squared_v2(v2)) - 1.0f) < 0.0001f || fabsf(test) < 0.0001f);
+#endif
+
 	/* this is the same as acos(dot_v3v3(v1, v2)), but more accurate */
 	if (dot_v2v2(v1, v2) < 0.0f) {
 		float vec[2];
@@ -408,6 +422,12 @@ void rotate_normalized_v3_v3v3fl(float r[3], const float p[3], const float axis[
 	const float costheta = cos(angle);
 	const float sintheta = sin(angle);
 
+	/* double check they are normalized */
+#ifdef DEBUG
+	float test;
+	BLI_assert(fabsf((test = len_squared_v3(axis)) - 1.0f) < 0.0001f || fabsf(test) < 0.0001f);
+#endif
+
 	r[0] = ((costheta + (1 - costheta) * axis[0] * axis[0]) * p[0]) +
 	       (((1 - costheta) * axis[0] * axis[1] - axis[2] * sintheta) * p[1]) +
 	       (((1 - costheta) * axis[0] * axis[2] + axis[1] * sintheta) * p[2]);