$ cat foo.go
package main

type I interface {
    DH()
}

type T struct{}

func (T) DH() {}
$ guru referrers foo.go:\#47
panic: assignment to entry in nil map

goroutine 1 [running]:
panic(0x778600, 0xc8213c6550)
    /usr/lib/go/src/runtime/panic.go:464 +0x3e6
main.globalReferrers(0xc8200b69a0, 0xc820012b70, 0xb, 0xc820012b70, 0xb, 0xc82000bcb0, 0x29, 0x2f, 0x7, 0x6, ...)
    /home/dominikh/prj/src/golang.org/x/tools/cmd/guru/referrers.go:178 +0x111
main.referrers(0xc8200b69a0, 0x0, 0x0)
    /home/dominikh/prj/src/golang.org/x/tools/cmd/guru/referrers.go:82 +0x96f
main.Run(0xc8200b69a0, 0x0, 0x0)
    /home/dominikh/prj/src/golang.org/x/tools/cmd/guru/guru.go:124 +0x637
main.main()
    /home/dominikh/prj/src/golang.org/x/tools/cmd/guru/main.go:204 +0x8d5