--- a/layout/painting/nsCSSRenderingBorders.cpp
+++ b/layout/painting/nsCSSRenderingBorders.cpp
@@ -1629,16 +1629,20 @@ nsCSSBorderRenderer::DrawBorderSides(int
 }
 
 void
 nsCSSBorderRenderer::SetupDashedOptions(StrokeOptions* aStrokeOptions,
                                         Float aDash[2],
                                         mozilla::Side aSide,
                                         Float aBorderLength, bool isCorner)
 {
+  MOZ_ASSERT(mBorderStyles[aSide] == NS_STYLE_BORDER_STYLE_DASHED ||
+             mBorderStyles[aSide] == NS_STYLE_BORDER_STYLE_DOTTED,
+             "Style should be dashed or dotted.");
+
   uint8_t style = mBorderStyles[aSide];
   Float borderWidth = mBorderWidths[aSide];
 
   // Dashed line starts and ends with half segment in most case.
   //
   // __--+---+---+---+---+---+---+---+---+--__
   //     |###|   |   |###|###|   |   |###|
   //     |###|   |   |###|###|   |   |###|