diff --git a/router/pcap.go b/router/pcap.go
index e8594ecd..b7365f68 100644
--- a/router/pcap.go
+++ b/router/pcap.go
@@ -2,6 +2,7 @@ package router
 
 import (
 	"code.google.com/p/gopacket/pcap"
+	"fmt"
 )
 
 type PcapIO struct {
@@ -44,6 +45,16 @@ func newPcapIO(ifName string, promisc bool, snaplen int, bufSz int) (handle *Pca
 		return
 	}
 	if err = inactive.SetImmediateMode(true); err != nil {
+		// If gopacket is compiled against an older pcap.h that
+		// doesn't have pcap_set_immediate_mode, it supplies a dummy
+		// definition that always returns PCAP_ERROR.  That becomes
+		// "Generic error", which is not very helpful.  The real
+		// pcap_set_immediate_mode never returns PCAP_ERROR, so this
+		// turns it into a more informative message.
+		if fmt.Sprint(err) == "Generic error" {
+			err = fmt.Errorf("compiled against an old version of libpcap; please compile against libpcap-1.5.0 or later")
+		}
+
 		return
 	}
 	if err = inactive.SetBufferSize(bufSz); err != nil {