diff --git a/scour/scour.py b/scour/scour.py
index 2fc4921..53bb36e 100644
--- a/scour/scour.py
+++ b/scour/scour.py
@@ -1750,7 +1750,7 @@ def removeDefaultAttributeValue(node, attribute):
             return 1
    else:
       nodeValue = SVGLength(node.getAttribute(attribute.name))
-      if (attribute.value is None) or (nodeValue.value == attribute.value):
+      if (attribute.value is None) or ((nodeValue.value == attribute.value) and not (nodeValue.units == Unit.INVALID)):
          if (attribute.units is None) or (nodeValue.units == attribute.units) or (isinstance(attribute.units, list) and nodeValue.units in attribute.units):
             if (attribute.conditions is None) or attribute.conditions(node):
                node.removeAttribute(attribute.name)
diff --git a/testscour.py b/testscour.py
index fa30bb6..52f243c 100755
--- a/testscour.py
+++ b/testscour.py
@@ -1238,11 +1238,19 @@ class DoNotCommonizeAttributesOnReferencedElements(unittest.TestCase):
 class DoNotRemoveOverflowVisibleOnMarker(unittest.TestCase):
 	def runTest(self):
 		doc = scour.scourXmlFile('unittests/overflow-marker.svg')
-		self.assertEqual(doc.getElementsByTagName('marker')[0].getAttribute('overflow'), 'visible',
+		self.assertEqual(doc.getElementById('m1').getAttribute('overflow'), 'visible',
 			'Removed the overflow attribute when it was not using the default value')
-		self.assertEqual(doc.getElementsByTagName('marker')[1].getAttribute('overflow'), '',
+		self.assertEqual(doc.getElementById('m2').getAttribute('overflow'), '',
 			'Did not remove the overflow attribute when it was using the default value')
 
+class DoNotRemoveOrientAutoOnMarker(unittest.TestCase):
+	def runTest(self):
+		doc = scour.scourXmlFile('unittests/orient-marker.svg')
+		self.assertEqual(doc.getElementById('m1').getAttribute('orient'), 'auto',
+			'Removed the orient attribute when it was not using the default value')
+		self.assertEqual(doc.getElementById('m2').getAttribute('orient'), '',
+			'Did not remove the orient attribute when it was using the default value')
+
 class MarkerOnSvgElements(unittest.TestCase):
 	def runTest(self):
 		doc = scour.scourXmlFile('unittests/overflow-svg.svg')
diff --git a/unittests/orient-marker.svg b/unittests/orient-marker.svg
new file mode 100644
index 0000000..19ecd19
--- /dev/null
+++ b/unittests/orient-marker.svg
@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+ <defs>
+  <marker id="m1" orient="auto">
+   <rect width="200" height="100"/>
+  </marker>
+  <marker id="m2" orient="0">
+   <rect width="200" height="100"/>
+  </marker>
+ </defs>
+ <line x2="100" y2="100" style="marker-start:url(#m1);marker-end:url(#m2)" stroke="#000" />
+</svg>