runtime error: invalid memory address or nil pointer dereference goroutine

runtime error: invalid memory address or nil pointer dereference
goroutine 4581 [running]:
net/http.(*conn).serve.func1(0xc000912000)
/usr/local/go/src/net/http/server.go:1769 +0x139
panic(0x1b939c0, 0x361eec0)
/usr/local/go/src/runtime/panic.go:522 +0x1b5
net.networkNumberAndMask(0x0, 0xc0007fba08, 0x5b4988, 0x0, 0xc0007fb9d0, 0x447e17, 0x0)
/usr/local/go/src/net/ip.go:473 +0x26
net.(*IPNet).String(0x0, 0xc000750bb8, 0xd)
/usr/local/go/src/net/ip.go:523 +0x40
github.com/contiv/vpp/plugins/ipam.(*IPAM).GetIPAMConfigForJSON(0xc000548000, 0xc000c84ce0)
/root/go/src/github.com/contiv/vpp/plugins/ipam/ipam.go:1134 +0x57
github.com/contiv/vpp/plugins/ipnet.(*IPNet).ipamGetHandler.func1(0x21b68c0, 0xc0001677a0, 0xc000a3a600)
/root/go/src/github.com/contiv/vpp/plugins/ipnet/rest.go:49 +0x32c
net/http.HandlerFunc.ServeHTTP(0xc000483f60, 0x21b68c0, 0xc0001677a0, 0xc000a3a600)
/usr/local/go/src/net/http/server.go:1995 +0x44
github.com/contiv/vpp/vendor/github.com/gorilla/mux.(*Router).ServeHTTP(0xc0000e2e40, 0x21b68c0, 0xc0001677a0, 0xc00099b200)
/root/go/src/github.com/contiv/vpp/vendor/github.com/gorilla/mux/mux.go:212 +0xe3
net/http.serverHandler.ServeHTTP(0xc0008344e0, 0x21b68c0, 0xc0001677a0, 0xc00099b200)
/usr/local/go/src/net/http/server.go:2774 +0xa8
net/http.(*conn).serve(0xc000912000, 0x21c4080, 0xc000c06d80)
/usr/local/go/src/net/http/server.go:1878 +0x851
created by net/http.(*Server).Serve
/usr/local/go/src/net/http/server.go:2884 +0x2f4
time="2020-02-18 08:40:25.29624" level=debug msg="Memif socket dump completed, found 1 entries: map[/run/vpp/memif.sock:0]" loc="vpp1908/dump_interface_vppcalls.go(299)" logger=vpp-ifplugin
time="2020-02-18 08:40:25.30418" level=debug msg="Memif socket dump completed, found 1 entries: map[/run/vpp/memif.sock:0]" loc="vpp1908/dump_interface_vppcalls.go(299)" logger=vpp-ifplugin