diff --git a/Lib/booleanOperations/flatten.py b/Lib/booleanOperations/flatten.py
index 68e602e..6b5100f 100644
--- a/Lib/booleanOperations/flatten.py
+++ b/Lib/booleanOperations/flatten.py
@@ -147,7 +147,7 @@ def __init__(self, points=None, previousOnCurve=None, willBeReversed=False):
         self.scaledPreviousOnCurve = _scaleSinglePoint(previousOnCurve, scale=clipperScale)
         self.used = False
         self.flat = []
-        # if the bcps are equal to the oncurves convert the segment to a line segment.
+        # if the bcps are equal to the oncurves, or are aligned with them, convert the segment to a line segment.
         # otherwise this causes an error when flattening.
         if self.segmentType == "curve":
             if previousOnCurve == points[0].coordinates and points[1].coordinates == points[-1].coordinates:
@@ -162,6 +162,11 @@ def __init__(self, points=None, previousOnCurve=None, willBeReversed=False):
                 oncurve = points[-1]
                 oncurve.segmentType = "line"
                 self.points = points = [oncurve]
+            elif (_pointOnLine(previousOnCurve, points[-1].coordinates, points[0].coordinates) and
+                  _pointOnLine(previousOnCurve, points[-1].coordinates, points[1].coordinates)):
+                oncurve = points[-1]
+                oncurve.segmentType = "line"
+                self.points = points = [oncurve]
         # its a reversed segment the flat points will be set later on in the InputContour
         if willBeReversed:
             return