diff --git a/src/FileDownload.cpp b/src/FileDownload.cpp
index a9a9678f4a..9a643f2195 100644
--- a/src/FileDownload.cpp
+++ b/src/FileDownload.cpp
@@ -201,7 +201,7 @@ void FileTransfer::StartTransfer( TransferType type, const RString &sURL, const
 	}
 	*/
 
-	vsHeaders.push_back( "Content-Length: " + ssprintf("%d",sRequestPayload.size()) );
+	vsHeaders.push_back( "Content-Length: " + ssprintf("%zd",sRequestPayload.size()) );
 
 	RString sHeader;
 	FOREACH_CONST( RString, vsHeaders, h )