diff --git a/src/drivers/net/rtl8139.c b/src/drivers/net/rtl8139.c
index 5e37f9c1..4981329c 100644
--- a/src/drivers/net/rtl8139.c
+++ b/src/drivers/net/rtl8139.c
@@ -367,8 +367,10 @@ static void rtl_close ( struct net_device *netdev ) {
  */
 static int rtl_transmit ( struct net_device *netdev, struct pk_buff *pkb ) {
 	struct rtl8139_nic *rtl = netdev->priv;
-	int align;
-	int pad_len;
+	void *data;
+	size_t len;
+	size_t headroom;
+	signed int pad_len;
 
 	/* Check for space in TX ring */
 	if ( rtl->tx.pkb[rtl->tx.next] != NULL ) {
@@ -376,11 +378,18 @@ static int rtl_transmit ( struct net_device *netdev, struct pk_buff *pkb ) {
 		return -ENOBUFS;
 	}
 
-	/* Align packet data */
-	align = ( virt_to_bus ( pkb->data ) & 0x3 );
-	pkb_push ( pkb, align );
-	memmove ( pkb->data, pkb->data + align, pkb_len ( pkb ) - align );
-	pkb_unput ( pkb, align );
+	/* Move packet data to start of packet buffer.  This will both
+	 * align the data (since packet buffers are aligned to
+	 * PKB_ALIGN) and give us sufficient space for the
+	 * zero-padding
+	 */
+	data = pkb->data;
+	len = pkb_len ( pkb );
+	headroom = pkb_headroom ( pkb );
+	pkb_push ( pkb, headroom );
+	memmove ( pkb->data, data, len );
+	pkb_unput ( pkb, headroom );
+	assert ( ( virt_to_bus ( pkb->data ) & 0x3 ) == 0 );
 
 	/* Pad to minimum packet length */
 	pad_len = ( ETH_ZLEN - pkb_len ( pkb ) );