diff --git a/mozilla/netwerk/dns/src/nsIDNService.cpp b/mozilla/netwerk/dns/src/nsIDNService.cpp
index 8514e02ab66..5c8a8e6e09d 100644
--- a/mozilla/netwerk/dns/src/nsIDNService.cpp
+++ b/mozilla/netwerk/dns/src/nsIDNService.cpp
@@ -242,7 +242,7 @@ NS_IMETHODIMP nsIDNService::IsACE(const nsACString & input, PRBool *_retval)
 NS_IMETHODIMP nsIDNService::Normalize(const nsACString & input, nsACString & output)
 {
   // protect against bogus input
-  NS_ENSURE_ARG(IsUTF8(input));
+  NS_ENSURE_TRUE(IsUTF8(input), NS_ERROR_UNEXPECTED);
 
   nsAutoString outUTF16;
   nsresult rv = stringPrep(NS_ConvertUTF8toUTF16(input), outUTF16);