--- a/sys/netgraph/iface/ng_iface.c
+++ b/sys/netgraph/iface/ng_iface.c
@@ -287,11 +287,21 @@
 				   M_NETGRAPH, M_NOWAIT);
 		if (newarray == NULL)
 			return (ENOMEM);
-		bcopy(ng_iface_units, newarray,
-		    ng_iface_units_len * sizeof(*ng_iface_units));
+
+		/*
+		 * Never pass a NULL source to bcopy(): sys/sys/systm.h
+		 * maps bcopy() onto __builtin_memmove(), and GCC then
+		 * assumes the source pointer cannot be NULL (even for
+		 * length 0) and optimizes away later NULL tests on it,
+		 * which turned the kfree() NULL guard below into dead
+		 * code (first call has ng_iface_units == NULL).
+		 */
+		if (ng_iface_units != NULL) {
+			bcopy(ng_iface_units, newarray,
+			    ng_iface_units_len * sizeof(*ng_iface_units));
+			kfree(ng_iface_units, M_NETGRAPH);
+		}
 		for (i = ng_iface_units_len; i < newlen; i++)
 			newarray[i] = ~0;
-		if (ng_iface_units != NULL)
-			kfree(ng_iface_units, M_NETGRAPH);
 		ng_iface_units = newarray;
 		ng_iface_units_len = newlen;