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

goroutine 15 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0004d7360?}, {0xe56770?, 0xc0004d75c0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0004d7340?}, {0xe56770?, 0xc0004d75a0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0004d7320?}, {0xe56770?, 0xc0004d7580?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0004d7300?}, {0xe56770?, 0xc0004d7560?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0004d72e0?}, {0xe56770?, 0xc0004d7540?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc0004d7500, 0xc00009f950, {0xe56770, 0xc0004d72c0?}, {0xc00009f8b8, {0x1db72e0, 0x4, 0x4}, 0xc000e34e40, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x418408?, 0x0?, 0xc000c4bca0?, {0xe56770, 0xc0004d72c0}, {0xc00009f8b8, {0x1db72e0, 0x4, 0x4}, 0xc000e34e40, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc000c4be48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc000e7a000, {0xe52aa8?, 0xc0015ca2a0}, 0x1e, 0xc001acc040, 0xc00007af00, {0xc001964000, 0x45c, 0x500})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc001acc640)
	/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