diff --git a/src/cmd/fmt.c b/src/cmd/fmt.c
index e43731e..59c1413 100644
--- a/src/cmd/fmt.c
+++ b/src/cmd/fmt.c
@@ -175,7 +175,7 @@
 	n = strlen(s);
 	if(n < 2)
 		return 1;
-	if(isupper(s[0]) && n < 4)
+	if(isupper((uchar)s[0]) && n < 4)
 		return 1;
 	if(strchr(".!?", s[n-1]) != nil)
 		return 2;