diff --git a/gfx/thebes/gfxUserFontSet.cpp b/gfx/thebes/gfxUserFontSet.cpp
index 487efac812b..9d4eb99cc84 100644
--- a/gfx/thebes/gfxUserFontSet.cpp
+++ b/gfx/thebes/gfxUserFontSet.cpp
@@ -129,14 +129,14 @@ gfxUserFontSet::AddFontFace(const nsAString& aFamilyName,
                             const nsString& aLanguageOverride,
                             gfxSparseBitSet *aUnicodeRanges)
 {
+    MOZ_ASSERT(aWeight != 0,
+               "aWeight must not be 0; use NS_FONT_WEIGHT_NORMAL instead");
+
     nsAutoString key(aFamilyName);
     ToLowerCase(key);
 
     bool found;
 
-    if (aWeight == 0)
-        aWeight = NS_FONT_WEIGHT_NORMAL;
-
     // stretch, italic/oblique ==> zero implies normal
 
     gfxMixedFontFamily *family = mFontFamilies.GetWeak(key, &found);
diff --git a/gfx/thebes/gfxUserFontSet.h b/gfx/thebes/gfxUserFontSet.h
index 446e236ee88..2424ef219f2 100644
--- a/gfx/thebes/gfxUserFontSet.h
+++ b/gfx/thebes/gfxUserFontSet.h
@@ -167,7 +167,7 @@ public:
 
 
     // add in a font face
-    // weight - 0 == unknown, [100, 900] otherwise (multiples of 100)
+    // weight - [100, 900] (multiples of 100)
     // stretch = [NS_FONT_STRETCH_ULTRA_CONDENSED, NS_FONT_STRETCH_ULTRA_EXPANDED]
     // italic style = constants in gfxFontConstants.h, e.g. NS_FONT_STYLE_NORMAL
     // TODO: support for unicode ranges not yet implemented
diff --git a/layout/style/nsFontFaceLoader.cpp b/layout/style/nsFontFaceLoader.cpp
index fa7e479bdfe..3e94cfce56a 100644
--- a/layout/style/nsFontFaceLoader.cpp
+++ b/layout/style/nsFontFaceLoader.cpp
@@ -578,6 +578,9 @@ nsUserFontSet::InsertRule(nsCSSFontFaceRule* aRule, uint8_t aSheetType,
   unit = val.GetUnit();
   if (unit == eCSSUnit_Integer || unit == eCSSUnit_Enumerated) {
     weight = val.GetIntValue();
+    if (weight == 0) {
+      weight = NS_STYLE_FONT_STYLE_NORMAL;
+    }
   } else if (unit == eCSSUnit_Normal) {
     weight = NS_STYLE_FONT_WEIGHT_NORMAL;
   } else {