diff --git a/src/path/PathItem.Boolean.js b/src/path/PathItem.Boolean.js
index 3472678f..c8a470f9 100644
--- a/src/path/PathItem.Boolean.js
+++ b/src/path/PathItem.Boolean.js
@@ -774,7 +774,7 @@ PathItem.inject(new function() {
         function isValid(seg) {
             var winding;
             return !!(seg && !seg._visited && (!operator
-                    || operator[(winding = seg._winding).winding]
+                    || operator[(winding = seg._winding || {}).winding]
                         // Unite operations need special handling of segments
                         // with a winding contribution of two (part of both
                         // areas), which are only valid if they are part of the