diff --git a/mailnews/mime/src/nsMsgHeaderParser.cpp b/mailnews/mime/src/nsMsgHeaderParser.cpp
index a13c8185cac..80e6546e467 100644
--- a/mailnews/mime/src/nsMsgHeaderParser.cpp
+++ b/mailnews/mime/src/nsMsgHeaderParser.cpp
@@ -192,10 +192,7 @@ NS_IMETHODIMP nsMsgHeaderParser::ParseHeadersWithArray(const PRUnichar * aLine,
 
 nsresult nsMsgHeaderParser::ParseHeaderAddresses (const char *charset, const char *line, char **names, char **addresses, PRUint32 *numAddresses)
 {
-#if DEBUG
-  if (line)
-    (void) NS_ConvertUTF8toUCS2(line).get(); // asserts if invalid UTF-8
-#endif
+  NS_ASSERTION(!line || IsUTF8(nsDependentCString(line)), "invalid UTF-8 in header");
 
   *numAddresses = msg_parse_Header_addresses(line, names, addresses);
 
@@ -206,10 +203,7 @@ nsresult nsMsgHeaderParser::ExtractHeaderAddressMailboxes (const char *charset,
 {
   if (mailboxes)
   {
-#if DEBUG
-    if (line)
-      (void) NS_ConvertUTF8toUCS2(line).get(); // asserts if invalid UTF-8
-#endif
+    NS_ASSERTION(!line || IsUTF8(nsDependentCString(line)), "invalid UTF-8 in header");
     
     *mailboxes = msg_extract_Header_address_mailboxes(line);
     return NS_OK;
@@ -222,10 +216,7 @@ nsresult nsMsgHeaderParser::ExtractHeaderAddressNames (const char *charset, cons
 {
   if (names)
   {
-#if DEBUG
-    if (line)
-      (void) NS_ConvertUTF8toUCS2(line).get(); // asserts if invalid UTF-8
-#endif
+    NS_ASSERTION(!line || IsUTF8(nsDependentCString(line)), "invalid UTF-8 in header");
 
     *names = msg_extract_Header_address_names(line);
     return NS_OK;
@@ -239,10 +230,7 @@ nsresult nsMsgHeaderParser::ExtractHeaderAddressName (const char *charset, const
 {
   if (name)
   {
-#if DEBUG
-    if (line)
-      (void) NS_ConvertUTF8toUCS2(line).get(); // asserts if invalid UTF-8
-#endif
+    NS_ASSERTION(!line || IsUTF8(nsDependentCString(line)), "invalid UTF-8 in header");
 
     *name = msg_extract_Header_address_name(line);
     return NS_OK;
@@ -255,10 +243,7 @@ nsresult nsMsgHeaderParser::ReformatHeaderAddresses (const char *charset, const
 {
   if (reformattedAddress)
   {
-#if DEBUG
-    if (line)
-      (void) NS_ConvertUTF8toUCS2(line).get(); // asserts if invalid UTF-8
-#endif
+    NS_ASSERTION(!line || IsUTF8(nsDependentCString(line)), "invalid UTF-8 in header");
 
     *reformattedAddress = msg_reformat_Header_addresses(line);
     return NS_OK;
@@ -271,12 +256,8 @@ nsresult nsMsgHeaderParser::RemoveDuplicateAddresses (const char *charset, const
 {
   if (newOutput)
   {
-#if DEBUG
-    if (addrs)
-      (void) NS_ConvertUTF8toUCS2(addrs).get(); // asserts if invalid UTF-8
-    if (other_addrs)
-      (void) NS_ConvertUTF8toUCS2(other_addrs).get();
-#endif
+    NS_ASSERTION(!addrs || IsUTF8(nsDependentCString(addrs)), "invalid UTF-8 in header");
+    NS_ASSERTION(!other_addrs || IsUTF8(nsDependentCString(other_addrs)), "invalid UTF-8 in header");
 
     *newOutput = msg_remove_duplicate_addresses(addrs, other_addrs, removeAliasesToMe);
     return NS_OK;
@@ -288,11 +269,7 @@ nsresult nsMsgHeaderParser::RemoveDuplicateAddresses (const char *charset, const
 nsresult nsMsgHeaderParser::MakeFullAddress (const char *charset, const char* name, const char* addr, char ** fullAddress)
 {
   NS_ENSURE_ARG_POINTER(fullAddress);
-
-#if DEBUG
-  if (addr)
-    (void) NS_ConvertUTF8toUCS2(addr).get(); // asserts if invalid UTF-8
-#endif
+  NS_ASSERTION(!addr || IsUTF8(nsDependentCString(addr)), "invalid UTF-8 in header");
   
   *fullAddress = msg_make_full_address(name, addr);
   return NS_OK;
@@ -316,12 +293,7 @@ nsresult nsMsgHeaderParser::MakeFullAddressWString (const PRUnichar* name, const
 nsresult nsMsgHeaderParser::UnquotePhraseOrAddr (const char *line, PRBool preserveIntegrity, char** result)
 {
   NS_ENSURE_ARG_POINTER(result);
-
-#if DEBUG
-  if (line)
-    (void) NS_ConvertUTF8toUCS2(line).get(); // asserts if invalid UTF-8
-#endif
-
+  NS_ASSERTION(!line || IsUTF8(nsDependentCString(line)), "invalid UTF-8 in header");
   return msg_unquote_phrase_or_addr(line, preserveIntegrity, result);
 }