diff --git a/gfx/src/xprint/nsFontMetricsXP.cpp b/gfx/src/xprint/nsFontMetricsXP.cpp
index eefda3f84962..3da78442b882 100644
--- a/gfx/src/xprint/nsFontMetricsXP.cpp
+++ b/gfx/src/xprint/nsFontMetricsXP.cpp
@@ -2675,7 +2675,7 @@ nsFontMetricsXP::FindGenericFont(nsFontSearch* aSearch)
   char name[128];
   if (mLangGroup) {
     nsAutoString pref = prefix;
-    pref.Append('.');
+    pref.AppendWithConversion('.');
     const PRUnichar* langGroup = nsnull;
     mLangGroup->GetUnicode(&langGroup);
     pref.Append(langGroup);