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

goroutine 10 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0007bdc40?}, {0xe56770?, 0xc0007bdda0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0007bdc20?}, {0xe56770?, 0xc0007bdd80?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0007bdc00?}, {0xe56770?, 0xc0007bdd60?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0007bdbe0?}, {0xe56770?, 0xc0007bdd40?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0007bdbc0?}, {0xe56770?, 0xc0007bdd20?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc0007bdce0, 0xc0015fad20, {0xe56770, 0xc0007bdba0?}, {0xc0015facd8, {0x1db72e0, 0x4, 0x4}, 0xc0016a6090, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x418408?, 0xc0003b2600?, 0xc00ccd5dc8?, {0xe56770, 0xc0007bdba0}, {0xc0015facd8, {0x1db72e0, 0x4, 0x4}, 0xc0016a6090, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc00ccd5e48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc0015fdfc0, {0xe52aa8?, 0xc0017d42a0}, 0x1e, 0xc0014da080, 0xc0000a3e90, {0xc0000ce000, 0x580, 0x700})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc0014da800)
	/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