addSelf takes a vector not positions.

src/extras/core/CurvePath.js

@@ -167,13 +167,10 @@ THREE.CurvePath.prototype.getBoundingBox = function () {
 			if ( p.z > maxZ ) maxZ = p.z;
 			else if ( p.z < minZ ) minZ = p.z;
 
-			sum.addSelf( p.x, p.y, p.z );
-
-		} else {
+		}
 
-			sum.addSelf( p.x, p.y );
+		sum.addSelf( p );
 
-		}
 	}
 
 	var ret = {