Index: sys/dev/e1000/em_txrx.c
===================================================================
--- sys/dev/e1000/em_txrx.c
+++ sys/dev/e1000/em_txrx.c
@@ -458,7 +458,13 @@
 	ntxd = scctx->isc_ntxd[0];
 	do {
 		delta = (int32_t)cur - (int32_t)prev;
+		/*
+		 * XXX This appears to be a hack for first-packet.
+		 * A correct fix would prevent prev == cur in the first place.
+		 */
 		MPASS(prev == 0 || delta != 0);
+		if (prev == 0 && cur == 0)
+			delta += 1;
 		if (delta < 0)
 			delta += ntxd;
 		DPRINTF(iflib_get_dev(adapter->ctx),
Index: sys/dev/e1000/igb_txrx.c
===================================================================
--- sys/dev/e1000/igb_txrx.c
+++ sys/dev/e1000/igb_txrx.c
@@ -333,7 +333,13 @@
 	ntxd = scctx->isc_ntxd[0];
 	do {
 		delta = (int32_t)cur - (int32_t)prev;
+		/*
+		 * XXX This appears to be a hack for first-packet.
+		 * A correct fix would prevent prev == cur in the first place.
+		 */
 		MPASS(prev == 0 || delta != 0);
+		if (prev == 0 && cur == 0)
+			delta += 1;
 		if (delta < 0)
 			delta += ntxd;