diff --git a/src/path/Curve.js b/src/path/Curve.js
index 759d1313..b500849d 100644
--- a/src/path/Curve.js
+++ b/src/path/Curve.js
@@ -301,11 +301,6 @@ var Curve = Base.extend(/** @lends Curve# */{
                 && this._segment2._handleIn.isZero();
     },
 
-    isHorizontal: function() {
-        return this.isLinear() && Numerical.isZero(
-                this._segment1._point._y - this._segment2._point._y);
-    },
-
     // DOCS: Curve#getIntersections()
     getIntersections: function(curve) {
         return Curve.filterIntersections(Curve.getIntersections(
diff --git a/src/path/PathItem.Boolean.js b/src/path/PathItem.Boolean.js
index 381c1bdb..bacd6543 100644
--- a/src/path/PathItem.Boolean.js
+++ b/src/path/PathItem.Boolean.js
@@ -124,7 +124,10 @@ PathItem.inject(new function() {
                             length = curveLength / 2;
                         var curve = node.segment.getCurve(),
                             pt = curve.getPointAt(length),
-                            hor = curve.isHorizontal(),
+                            // Determine if the curve is a horizontal linear
+                            // curve by checking the slope of it's tangent.
+                            hor = curve.isLinear() && Math.abs(curve
+                                    .getTangentAt(0.5, true).y) <= tolerance,
                             path = curve._path;
                         if (path._parent instanceof CompoundPath)
                             path = path._parent;