diff --git a/mailnews/compose/src/nsMsgCompFields.cpp b/mailnews/compose/src/nsMsgCompFields.cpp
index e958c9ae2ef..7d85156a36f 100644
--- a/mailnews/compose/src/nsMsgCompFields.cpp
+++ b/mailnews/compose/src/nsMsgCompFields.cpp
@@ -605,7 +605,7 @@ nsresult nsMsgCompFields::DecodeHeader(MSG_HEADER_SET header)
 		break;
     }
 
-    NS_ASSERTION(result < sizeof(m_headers) / sizeof(char*), "wrong result, review the code!");
+    NS_ASSERTION(result < (int)(sizeof(m_headers) / sizeof(char*)), "wrong result, review the code!");
     return result;
 }