diff --git a/scapy/layers/l2.py b/scapy/layers/l2.py
index d11bb8dac549e975dde3d8453d2a67b29a6071bf..26f7871c75975fb036c18f7571f9fd0ab35cd09b 100644
--- a/scapy/layers/l2.py
+++ b/scapy/layers/l2.py
@@ -103,7 +103,9 @@ class SourceMACField(MACField):
         MACField.__init__(self, name, None)
     def i2h(self, pkt, x):
         if x is None:
-            iff,a,gw = pkt.payload.route()
+            iff, a, gw = pkt.payload.route()
+            if iff is None:
+                iff = conf.iface
             if iff:
                 try:
                     x = get_if_hwaddr(iff)