diff --git a/xpcom/build/dlldeps.cpp b/xpcom/build/dlldeps.cpp
index 73393bc5be07..3028a84b5c32 100644
--- a/xpcom/build/dlldeps.cpp
+++ b/xpcom/build/dlldeps.cpp
@@ -197,6 +197,6 @@ void XXXNeverCalled()
       nsAdoptingString foo, bar;
       foo = bar;
     }
-    NS_UTF16ToCString(str1, 0, str2);
-    NS_CStringToUTF16(str2, 0, str1);
+    NS_UTF16ToCString(str1, NS_CSTRING_ENCODING_ASCII, str2);
+    NS_CStringToUTF16(str2, NS_CSTRING_ENCODING_ASCII, str1);
 }