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

goroutine 12 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000820fa0?}, {0xe56770?, 0xc0008213a0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000820f80?}, {0xe56770?, 0xc000821380?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000820f60?}, {0xe56770?, 0xc000821360?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000820f40?}, {0xe56770?, 0xc000821340?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000820f20?}, {0xe56770?, 0xc000821320?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc000821040, 0xc00c9f2d70, {0xe56770, 0xc000820f00?}, {0xc00c9f2c88, {0x1db72e0, 0x4, 0x4}, 0xc00c9f9290, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x0?, 0xc00039a480?, 0xc00b65fdc8?, {0xe56770, 0xc000820f00}, {0xc00c9f2c88, {0x1db72e0, 0x4, 0x4}, 0xc00c9f9290, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc00b65fe48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc00cad8600, {0xe52aa8?, 0xc001200300}, 0x1e, 0xc0018b4000, 0xc000113e90, {0xc001654000, 0x40c, 0x500})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc001999b00)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:95 +0x365
created by main.main in goroutine 1
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/fuzzer.go:336 +0x1665