diff --git a/src/math/Vector3.js b/src/math/Vector3.js
index e83347e73f169665f315c153cb110fe553c264ce..d1be1894edd6bcdbde50b61c8072d911d3bd95cf 100644
--- a/src/math/Vector3.js
+++ b/src/math/Vector3.js
@@ -564,7 +564,17 @@ THREE.extend( THREE.Vector3.prototype, {
 
 	angleTo: function ( v ) {
 
-		return Math.acos( Math.min ( 1, this.dot( v ) / this.length() / v.length() ) );
+		// clamp, to handle numerical problems
+
+		function clamp( x ) {
+
+			return Math.min( Math.max( x, -1 ), 1 );
+
+		}
+
+		var theta = this.dot( v ) / ( this.length() * v.length() );
+
+		return Math.acos( clamp ( theta ) );
 
 	},
 
diff --git a/test/unit/math/Vector3.js b/test/unit/math/Vector3.js
index 176a2dab219f88725b71272f9d7ebe219e39b3da..2aa2370edde6f45df2804b64ed5d7ee822628602 100644
--- a/test/unit/math/Vector3.js
+++ b/test/unit/math/Vector3.js
@@ -292,8 +292,10 @@ test( "reflect", function() {
 
 test( "angleTo", function() {
 	var a = new THREE.Vector3( 0, -0.18851655680720186, 0.9820700116639124 );
+	var b = new THREE.Vector3( 0, 0.18851655680720186, -0.9820700116639124 );
 
-	equal( a.angleTo( a ),  0 );
+	equal( a.angleTo( a ), 0 );
+	equal( a.angleTo( b ), Math.PI );
 
 	var x = new THREE.Vector3( 1, 0, 0 );
 	var y = new THREE.Vector3( 0, 1, 0 );