diff --git a/drivers/s390/net/cu3088.c b/drivers/s390/net/cu3088.c
index b12533104c1f..e1b8c9a9634f 100644
--- a/drivers/s390/net/cu3088.c
+++ b/drivers/s390/net/cu3088.c
@@ -77,7 +77,7 @@ group_write(struct device_driver *drv, const char *buf, size_t count)
 		int len;
 
 		if (!(end = strchr(start, delim[i])))
-			return count;
+			return -EINVAL;
 		len = min_t(ptrdiff_t, BUS_ID_SIZE, end - start + 1);
 		strlcpy (bus_ids[i], start, len);
 		argv[i] = bus_ids[i];