panic: replaceArg: group fields don't match: 1/0

goroutine 14 [running]:
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b21080?}, {0x9fabc8?, 0x4017878420?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x3fc
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b21320?}, {0x9fabc8?, 0x4017878400?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x328
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b21340?}, {0x9fabc8?, 0x40178783e0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x328
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b21360?}, {0x9fabc8?, 0x40178783c0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x328
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b21380?}, {0x9fabc8?, 0x40178783a0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x328
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b213a0?}, {0x9fabc8?, 0x4017878380?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x328
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b213c0?}, {0x9fabc8?, 0x4017878360?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x328
github.com/google/syzkaller/prog.replaceArg({0x9fabc8?, 0x4016b213e0?}, {0x9fabc8?, 0x4017878340?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x328
github.com/google/syzkaller/prog.(*StructType).mutate(0x1251ae0, 0x4016b20f00, 0x401762ed20, {0x9fabc8, 0x4016b21400?}, {0x401762eb48, {0x1914440, 0x4, 0x4}, 0x40176635f0, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x170
github.com/google/syzkaller/prog.(*Target).mutateArg(0x400a39a100?, 0x4017663170?, 0x4?, {0x9fabc8, 0x4016b21400}, {0x401762eb48, {0x1914440, 0x4, 0x4}, 0x40176635f0, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xc0
github.com/google/syzkaller/prog.(*mutator).mutateArg(0x4001cdfd48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x1f4
github.com/google/syzkaller/prog.(*Prog).Mutate(0x40189a3800, {0x9f6e48?, 0x4018c165d0}, 0x1e, 0x4017c8e000, 0x40003e2e70, {0x4015b82000, 0xde2f, 0x11000})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x224
main.(*Proc).smashInput(0x4017c8e0c0, 0x4011c81050)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:221 +0x10c
main.(*Proc).loop(0x4017c8e0c0)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:78 +0xf4
created by main.main in goroutine 1
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/fuzzer.go:336 +0x1288