diff --git a/cpp/clipper.cpp b/cpp/clipper.cpp

index 02a7b70..aa146ea 100644

--- a/cpp/clipper.cpp

+++ b/cpp/clipper.cpp

@@ -896,13 +896,14 @@ bool ClipperBase::AddPolygon(const Polygon &pg, PolyType polyType)

     RangeTest(pg[i], maxVal);

     if (i == 0 || PointsEqual(p[j], pg[i])) continue;

-    else if (j > 0 && SlopesEqual(p[j-1], p[j], pg[i], m_UseFullRange))

+    else if (j > 0 && SlopesEqual(p[j-1], p[j], pg[i], maxVal == hiRange))

     {

       if (PointsEqual(p[j-1], pg[i])) j--;

     } else j++;

     p[j] = pg[i];

   }

   if (j < 2) return false;

+  m_UseFullRange = (maxVal == hiRange);

   len = j+1;

   while (len > 2)