panic: replaceArg: group fields don't match: 1/2

goroutine 12 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00072ce20?}, {0xe56770?, 0xc00072d140?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00072ce00?}, {0xe56770?, 0xc00072d120?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00072cde0?}, {0xe56770?, 0xc00072d100?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00072cdc0?}, {0xe56770?, 0xc00072d0e0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00072cda0?}, {0xe56770?, 0xc00072d0c0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc00072cfc0, 0xc001354690, {0xe56770, 0xc00072cd80?}, {0xc001354558, {0x1db72e0, 0x4, 0x4}, 0xc00123bc20, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x7f8896e2dd00?, 0x7f88dd9e9108?, 0x10?, {0xe56770, 0xc00072cd80}, {0xc001354558, {0x1db72e0, 0x4, 0x4}, 0xc00123bc20, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc001123e48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc001683a80, {0xe52aa8?, 0xc0011b8de0}, 0x1e, 0xc0012f20c0, 0xc0000a3dd0, {0xc000cac000, 0x45d, 0x500})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc001668140)
	/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