diff --git a/core/src/processing/core/PVector.java b/core/src/processing/core/PVector.java
index 1fb4f1f76..59db99def 100644
--- a/core/src/processing/core/PVector.java
+++ b/core/src/processing/core/PVector.java
@@ -537,7 +537,16 @@ public class PVector {
     double dot = v1.x * v2.x + v1.y * v2.y + v1.z * v2.z;
     double v1mag = Math.sqrt(v1.x * v1.x + v1.y * v1.y + v1.z * v1.z);
     double v2mag = Math.sqrt(v2.x * v2.x + v2.y * v2.y + v2.z * v2.z);
-    return (float) Math.acos(dot / (v1mag * v2mag));
+    // This should be a number between -1 and 1, since it's "normalized"
+    double amt = dot / (v1mag * v2mag);
+    // But if it's not due to rounding error, then we need to fix it
+    // http://code.google.com/p/processing/issues/detail?id=340
+    // Otherwise if outside the range, acos() will return NaN 
+    // http://www.cppreference.com/wiki/c/math/acos
+    if (amt <= -1 || amt >= 1) {
+      return PConstants.PI;
+    }
+    return (float) Math.acos(amt);
   }
 
 
diff --git a/core/todo.txt b/core/todo.txt
index 639269b7c..0eecae3cf 100644
--- a/core/todo.txt
+++ b/core/todo.txt
@@ -2,6 +2,8 @@
 X fix background(PImage) for OpenGL
 X   http://code.google.com/p/processing/issues/detail?id=336
 X skip null entries with trim(String[]) 
+X NaN with PVector.angleBetween
+X   http://code.google.com/p/processing/issues/detail?id=340
 
 Hi all, just thought I'd share something. 
 For a while now, I've been having unexpected frame rate drops with JOGL when switching into fullscreen-exclusive mode (it halved no matter what I did - resolution switching, reducing how much I drew, etc).