diff --git a/dom/svg/SVGPathData.cpp b/dom/svg/SVGPathData.cpp
index 1bf819a1f44..0e11782754f 100644
--- a/dom/svg/SVGPathData.cpp
+++ b/dom/svg/SVGPathData.cpp
@@ -537,9 +537,9 @@ AngleOfVector(const Point& aVector)
 }
 
 static float
-AngleOfVectorF(const Point& aVector)
+AngleOfVector(const Point& cp1, const Point& cp2)
 {
-  return static_cast<float>(AngleOfVector(aVector));
+  return static_cast<float>(AngleOfVector(cp1 - cp2));
 }
 
 void
@@ -573,7 +573,7 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
     {
     case PATHSEG_CLOSEPATH:
       segEnd = pathStart;
-      segStartAngle = segEndAngle = AngleOfVectorF(segEnd - segStart);
+      segStartAngle = segEndAngle = AngleOfVector(segEnd, segStart);
       break;
 
     case PATHSEG_MOVETO_ABS:
@@ -586,7 +586,7 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
       pathStart = segEnd;
       // If authors are going to specify multiple consecutive moveto commands
       // with markers, me might as well make the angle do something useful:
-      segStartAngle = segEndAngle = AngleOfVectorF(segEnd - segStart);
+      segStartAngle = segEndAngle = AngleOfVector(segEnd, segStart);
       i += 2;
       break;
 
@@ -597,7 +597,7 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
       } else {
         segEnd = segStart + Point(mData[i], mData[i+1]);
       }
-      segStartAngle = segEndAngle = AngleOfVectorF(segEnd - segStart);
+      segStartAngle = segEndAngle = AngleOfVector(segEnd, segStart);
       i += 2;
       break;
 
@@ -615,14 +615,10 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
         segEnd = segStart + Point(mData[i+4], mData[i+5]);
       }
       prevCP = cp2;
-      if (cp1 == segStart) {
-        cp1 = cp2;
-      }
-      if (cp2 == segEnd) {
-        cp2 = cp1;
-      }
-      segStartAngle = AngleOfVectorF(cp1 - segStart);
-      segEndAngle = AngleOfVectorF(segEnd - cp2);
+      segStartAngle =
+        AngleOfVector(cp1 == segStart ? (cp1 == cp2 ? segEnd : cp2) : cp1, segStart);
+      segEndAngle =
+        AngleOfVector(segEnd, cp2 == segEnd ? (cp1 == cp2 ? segStart : cp1) : cp2);
       i += 6;
       break;
     }
@@ -630,7 +626,7 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
     case PATHSEG_CURVETO_QUADRATIC_ABS:
     case PATHSEG_CURVETO_QUADRATIC_REL:
     {
-      Point cp1, cp2; // control points
+      Point cp1; // control point
       if (segType == PATHSEG_CURVETO_QUADRATIC_ABS) {
         cp1 = Point(mData[i], mData[i+1]);
         segEnd = Point(mData[i+2], mData[i+3]);
@@ -639,8 +635,8 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
         segEnd = segStart + Point(mData[i+2], mData[i+3]);
       }
       prevCP = cp1;
-      segStartAngle = AngleOfVectorF(cp1 - segStart);
-      segEndAngle = AngleOfVectorF(segEnd - cp1);
+      segStartAngle = AngleOfVector(cp1 == segStart ? segEnd : cp1, segStart);
+      segEndAngle = AngleOfVector(segEnd, cp1 == segEnd ? segStart : cp1);
       i += 4;
       break;
     }
@@ -680,7 +676,7 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
 
       if (rx == 0.0 || ry == 0.0) {
         // F.6.6 step 1 - straight line or coincidental points
-        segStartAngle = segEndAngle = AngleOfVectorF(segEnd - segStart);
+        segStartAngle = segEndAngle = AngleOfVector(segEnd, segStart);
         i += 7;
         break;
       }
@@ -755,7 +751,7 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
       } else {
         segEnd = segStart + Point(mData[i++], 0.0f);
       }
-      segStartAngle = segEndAngle = AngleOfVectorF(segEnd - segStart);
+      segStartAngle = segEndAngle = AngleOfVector(segEnd, segStart);
       break;
 
     case PATHSEG_LINETO_VERTICAL_ABS:
@@ -765,7 +761,7 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
       } else {
         segEnd = segStart + Point(0.0f, mData[i++]);
       }
-      segStartAngle = segEndAngle = AngleOfVectorF(segEnd - segStart);
+      segStartAngle = segEndAngle = AngleOfVector(segEnd, segStart);
       break;
 
     case PATHSEG_CURVETO_CUBIC_SMOOTH_ABS:
@@ -782,14 +778,10 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
         segEnd = segStart + Point(mData[i+2], mData[i+3]);
       }
       prevCP = cp2;
-      if (cp1 == segStart) {
-        cp1 = cp2;
-      }
-      if (cp2 == segEnd) {
-        cp2 = cp1;
-      }
-      segStartAngle = AngleOfVectorF(cp1 - segStart);
-      segEndAngle = AngleOfVectorF(segEnd - cp2);
+      segStartAngle =
+        AngleOfVector(cp1 == segStart ? (cp1 == cp2 ? segEnd : cp2) : cp1, segStart);
+      segEndAngle =
+        AngleOfVector(segEnd, cp2 == segEnd ? (cp1 == cp2 ? segStart : cp1) : cp2);
       i += 4;
       break;
     }
@@ -799,15 +791,14 @@ SVGPathData::GetMarkerPositioningData(nsTArray<nsSVGMark> *aMarks) const
     {
       Point cp1 = SVGPathSegUtils::IsQuadraticType(prevSegType) ?
                        segStart * 2 - prevCP : segStart;
-      Point cp2;
       if (segType == PATHSEG_CURVETO_QUADRATIC_SMOOTH_ABS) {
         segEnd = Point(mData[i], mData[i+1]);
       } else {
         segEnd = segStart + Point(mData[i], mData[i+1]);
       }
       prevCP = cp1;
-      segStartAngle = AngleOfVectorF(cp1 - segStart);
-      segEndAngle = AngleOfVectorF(segEnd - cp1);
+      segStartAngle = AngleOfVector(cp1 == segStart ? segEnd : cp1, segStart);
+      segEndAngle = AngleOfVector(segEnd, cp1 == segEnd ? segStart : cp1);
       i += 2;
       break;
     }
diff --git a/layout/reftests/svg/marker-orientation-04.svg b/layout/reftests/svg/marker-orientation-04.svg
new file mode 100644
index 00000000000..3fb2fdd0b92
--- /dev/null
+++ b/layout/reftests/svg/marker-orientation-04.svg
@@ -0,0 +1,39 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<svg xmlns="http://www.w3.org/2000/svg">
+  <defs>
+    <marker id="redTriangle"
+      refX="0" refY="12" markerUnits="userSpaceOnUse"
+      markerWidth="24" markerHeight="24" orient="auto">
+      <path fill="red" d="M 0 0 L 24 12 L 0 24 z" />
+    </marker>
+    <marker id="limeTriangle"
+      refX="0" refY="12" markerUnits="userSpaceOnUse"
+      markerWidth="24" markerHeight="24" orient="auto" overflow="visible">
+      <path fill="lime" stroke-width="3" stroke="lime" d="M 0 0 L 24 12 L 0 24 z" />
+    </marker>
+    <marker id="reverseLimeTriangle"
+      refX="0" refY="12" markerUnits="userSpaceOnUse"
+      markerWidth="24" markerHeight="24" orient="auto" overflow="visible">
+      <path transform="rotate(180, 0, 12)" fill="lime" stroke-width="3" stroke="lime" d="M 0 0 L 24 12 L 0 24 z" />
+    </marker>
+  </defs>
+
+  <rect width="100%" height="100%" fill="lime"/>
+
+  <!-- Control point vectors are zero -->
+  <path d="M 50,125 C 50,125, 100,25, 100,25" stroke-width="10" stroke="red"
+    marker-end="url(#redTriangle)" />
+
+  <path d="M 150,125 C 200,25, 200,25, 200,25" stroke-width="10" stroke="red"
+    marker-end="url(#redTriangle)" />
+
+  <!-- Non-degenerate covering -->
+  <path d="M 50,125 L 100,25" stroke-width="13" stroke="lime"
+    marker-end="url(#limeTriangle)" marker-start="url(#reverseLimeTriangle)" />
+
+  <path d="M 150,125 L 200,25" stroke-width="13" stroke="lime"
+    marker-end="url(#limeTriangle)" marker-start="url(#reverseLimeTriangle)" />
+</svg>
diff --git a/layout/reftests/svg/reftest.list b/layout/reftests/svg/reftest.list
index 1c35d1478df..cf80bb59027 100644
--- a/layout/reftests/svg/reftest.list
+++ b/layout/reftests/svg/reftest.list
@@ -201,6 +201,7 @@ fuzzy-if(cocoaWidget,15,19679) fuzzy-if(winWidget,1,8800) fuzzy-if(!cocoaWidget&
 == marker-orientation-01.svg marker-orientation-01-ref.svg
 pref(svg.marker-improvements.enabled,true) == marker-orientation-02.svg marker-orientation-02-ref.svg
 == marker-orientation-03.svg pass.svg
+== marker-orientation-04.svg pass.svg
 == mask-basic-01.svg pass.svg
 == mask-basic-02.svg mask-basic-02-ref.svg
 == mask-basic-03.svg pass.svg