diff --git a/src/http_htx.c b/src/http_htx.c
index 6c288f9f5..399848db6 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -1760,7 +1760,7 @@ int http_scheme_based_normalize(struct htx *htx)
 	authority = http_get_authority(uri, 0);
 	start = istptr(authority);
 	end = istend(authority);
-	for (ptr = end; ptr > start && isdigit(*--ptr); )
+	for (ptr = end; ptr > start && isdigit((unsigned char)*--ptr); )
 		;
 
 	/* if no port found, no normalization to proceed */