diff --git a/layout/generic/nsImageMap.cpp b/layout/generic/nsImageMap.cpp
index 19c8797567..bc200a345d 100644
--- a/layout/generic/nsImageMap.cpp
+++ b/layout/generic/nsImageMap.cpp
@@ -529,7 +529,7 @@ void PolyArea::Draw(nsIFrame* aFrame, DrawTarget& aDrawTarget,
       Point p1(pc->CSSPixelsToDevPixels(mCoords[0]),
                pc->CSSPixelsToDevPixels(mCoords[1]));
       Point p2, p1snapped, p2snapped;
-      for (int32_t i = 2; i < mNumCoords; i += 2) {
+      for (int32_t i = 2; i < mNumCoords - 1; i += 2) {
         p2.x = pc->CSSPixelsToDevPixels(mCoords[i]);
         p2.y = pc->CSSPixelsToDevPixels(mCoords[i+1]);
         p1snapped = p1;
@@ -556,7 +556,7 @@ void PolyArea::GetRect(nsIFrame* aFrame, nsRect& aRect)
     nscoord x1, x2, y1, y2, xtmp, ytmp;
     x1 = x2 = nsPresContext::CSSPixelsToAppUnits(mCoords[0]);
     y1 = y2 = nsPresContext::CSSPixelsToAppUnits(mCoords[1]);
-    for (int32_t i = 2; i < mNumCoords; i += 2) {
+    for (int32_t i = 2; i < mNumCoords - 1; i += 2) {
       xtmp = nsPresContext::CSSPixelsToAppUnits(mCoords[i]);
       ytmp = nsPresContext::CSSPixelsToAppUnits(mCoords[i+1]);
       x1 = x1 < xtmp ? x1 : xtmp;