--- lib/x509/crl_write.c
+++ lib/x509/crl_write.c
@@ -60,7 +60,7 @@
 gnutls_x509_crl_set_version (gnutls_x509_crl_t crl, unsigned int version)
 {
   int result;
-  char null = version;
+  signed char null = version;
 
   if (crl == NULL)
     {