diff --git a/drivers/staging/r8188eu/os_dep/ioctl_linux.c b/drivers/staging/r8188eu/os_dep/ioctl_linux.c
index 1b09462ca908..8dd280e2739a 100644
--- a/drivers/staging/r8188eu/os_dep/ioctl_linux.c
+++ b/drivers/staging/r8188eu/os_dep/ioctl_linux.c
@@ -403,7 +403,7 @@ static int wpa_set_encryption(struct net_device *dev, struct ieee_param *param,
 
 		if (wep_key_len > 0) {
 			wep_key_len = wep_key_len <= 5 ? 5 : 13;
-			wep_total_len = wep_key_len + FIELD_OFFSET(struct ndis_802_11_wep, KeyMaterial);
+			wep_total_len = wep_key_len + sizeof(*pwep);
 			pwep = kzalloc(wep_total_len, GFP_KERNEL);
 			if (!pwep)
 				goto exit;