diff --git a/lib/asn1/hash.c b/lib/asn1/hash.c
index 1ccf319e8..c12d994cf 100644
--- a/lib/asn1/hash.c
+++ b/lib/asn1/hash.c
@@ -183,7 +183,7 @@ hashcaseadd(const char *s)
     assert(s);
 
     for (i = 0; *s; ++s)
-	i += toupper(*s);
+	i += toupper((unsigned char)*s);
     return i;
 }