panic: broken type ref

goroutine 37 [running]:
github.com/google/syzkaller/prog.ArgCommon.Type(...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:69
github.com/google/syzkaller/prog.isDefault({0xd28b60, 0xc00074f080})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:345 +0x22
github.com/google/syzkaller/prog.(*GroupArg).serialize(0xc000173780, 0xc0007cbd70)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/encoding.go:186 +0x2f7
github.com/google/syzkaller/prog.(*serializer).arg(0xd1ece0?, {0xd28c60?, 0xc000173780?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/encoding.go:121 +0x29
github.com/google/syzkaller/prog.(*PointerArg).serialize(0xc00064e3f0, 0xc0007cbd70)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/encoding.go:142 +0x38e
github.com/google/syzkaller/prog.(*serializer).arg(0xd1ece0?, {0xd28ba0?, 0xc00064e3f0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/encoding.go:121 +0x29
github.com/google/syzkaller/prog.(*serializer).call(0xc0007cbd70, 0xc00064afa0)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/encoding.go:82 +0x211
github.com/google/syzkaller/prog.(*Prog).serialize(0xc0006426c0, 0x0)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/encoding.go:46 +0xd8
github.com/google/syzkaller/prog.(*Prog).Serialize(...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/encoding.go:30
main.(*Proc).logProgram(0xc0000fc280, 0xc0006426c0)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:130 +0x4e
main.(*Proc).executeRaw(0xc0000fc280, 0xc0003d5ef0, 0xc0006426c0)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:101 +0x125
main.(*Proc).loop(0xc0000fc280)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:65 +0x148
created by main.main in goroutine 1
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/fuzzer.go:256 +0x118a