diff --git a/sys/src/libdisk/proto.c b/sys/src/libdisk/proto.c
--- a/sys/src/libdisk/proto.c
+++ b/sys/src/libdisk/proto.c
@@ -537,7 +537,10 @@ loop:
 		goto loop;
 	p--;
 	popopt(mkaux);
-	*strchr(p, '\n') = 0;
+	s = strchr(p, '\n');
+	if(s == nil)
+		return nil;
+	*s = 0;
 	if(s = strchr(p, '=')){
 		*s++ = 0;
 		setopt(mkaux, p, s);