diff --git a/layout/mathml/nsMathMLmencloseFrame.cpp b/layout/mathml/nsMathMLmencloseFrame.cpp
--- a/layout/mathml/nsMathMLmencloseFrame.cpp
+++ b/layout/mathml/nsMathMLmencloseFrame.cpp
@@ -794,17 +794,17 @@ void nsDisplayNotation::Paint(nsDisplayL
       gfxCtx->NewPath();
       gfxPoint p[] = {
         rect.TopRight(),
         rect.TopRight() + gfxPoint(-w -.4*h, std::max(-e / 2.0, h - .4*w)),
         rect.TopRight() + gfxPoint(-.7*w, .7*h),
         rect.TopRight() + gfxPoint(std::min(e / 2.0, -w + .4*h), h + .4*w),
         rect.TopRight()
       };
-      gfxCtx->Polygon(p, sizeof(p));
+      gfxCtx->Polygon(p, MOZ_ARRAY_LENGTH(p));
       gfxCtx->Fill();
     }
       break;
 
     default:
       NS_NOTREACHED("This notation can not be drawn using nsDisplayNotation");
       break;
     }

