diff --git a/mozilla/layout/mathml/base/src/nsMathMLChar.cpp b/mozilla/layout/mathml/base/src/nsMathMLChar.cpp
index 0ff3bf3f919..6d93ee77944 100644
--- a/mozilla/layout/mathml/base/src/nsMathMLChar.cpp
+++ b/mozilla/layout/mathml/base/src/nsMathMLChar.cpp
@@ -113,7 +113,7 @@ public:
 
   void GetFontName(nsString& aFontName)
   {
-    aFontName.Assign(mFontName);
+    aFontName.AssignWithConversion(mFontName);
   }
 
   nsGlyphTable* GetNextTable(void)