diff --git a/library/oid.c b/library/oid.c
index 891d3cd..e0c0743 100644
--- a/library/oid.c
+++ b/library/oid.c
@@ -761,7 +761,7 @@
         if( !( oid->p[i] & 0x80 ) )
         {
             /* Last byte */
-            ret = mbedtls_snprintf( p, n, ".%d", value );
+            ret = mbedtls_snprintf( p, n, ".%u", value );
             OID_SAFE_SNPRINTF;
             value = 0;
         }