--- a/layout/style/nsCSSProps.cpp
+++ b/layout/style/nsCSSProps.cpp
@@ -2286,18 +2286,20 @@ nsCSSProps::FindKeyword(nsCSSKeyword aKe
     return true;
   }
   return false;
 }
 
 nsCSSKeyword
 nsCSSProps::ValueToKeywordEnum(int32_t aValue, const KTableEntry aTable[])
 {
+#ifdef DEBUG
   typedef decltype(aTable[0].mValue) table_value_type;
   NS_ASSERTION(table_value_type(aValue) == aValue, "Value out of range");
+#endif
   for (int32_t i = 0; ; ++i) {
     const KTableEntry& entry = aTable[i];
     if (::IsKeyValSentinel(entry)) {
       break;
     }
     if (aValue == entry.mValue) {
       return entry.mKeyword;
     }
@@ -2328,18 +2330,20 @@ nsCSSProps::kKeywordTableTable[eCSSPrope
   #undef CSS_PROP
 };
 
 const nsAFlatCString&
 nsCSSProps::LookupPropertyValue(nsCSSProperty aProp, int32_t aValue)
 {
   MOZ_ASSERT(aProp >= 0 && aProp < eCSSProperty_COUNT,
              "property out of range");
+#ifdef DEBUG
   typedef decltype(KTableEntry::mValue) table_value_type;
   NS_ASSERTION(table_value_type(aValue) == aValue, "Value out of range");
+#endif
 
   const KTableEntry* kwtable = nullptr;
   if (aProp < eCSSProperty_COUNT_no_shorthands)
     kwtable = kKeywordTableTable[aProp];
 
   if (kwtable)
     return ValueToKeyword(aValue, kwtable);