diff --git a/sys/src/cmd/mothra/forms.c b/sys/src/cmd/mothra/forms.c
index 719720c1a..b3bbc2e9b 100644
--- a/sys/src/cmd/mothra/forms.c
+++ b/sys/src/cmd/mothra/forms.c
@@ -714,7 +714,7 @@ int Ufmt(Fmt *f){
 		else if(*s==' ')
 			fmtprint(f, "+");
 		else
-			fmtprint(f, "%%%.2X", (unsigned int)*s);
+			fmtprint(f, "%%%.2X", *s & 0xFF);
 	}
 	return 0;
 }