panic: replaceArg: group fields don't match: 0/16

goroutine 25 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891b80?}, {0xe56770?, 0xc000891ce0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891b40?}, {0xe56770?, 0xc000891ca0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891b20?}, {0xe56770?, 0xc000891c80?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891b00?}, {0xe56770?, 0xc000891c60?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891ae0?}, {0xe56770?, 0xc000891c40?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891ac0?}, {0xe56770?, 0xc000891c20?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891aa0?}, {0xe56770?, 0xc000891c00?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000891a80?}, {0xe56770?, 0xc000891be0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc000891ba0, 0xc001405180, {0xe56770, 0xc000891a60?}, {0xc001405138, {0x1db72e0, 0x4, 0x4}, 0xc00171a390, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x418408?, 0xc000378140?, 0xc000927dc8?, {0xe56770, 0xc000891a60}, {0xc001405138, {0x1db72e0, 0x4, 0x4}, 0xc00171a390, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc000927e48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc001715e80, {0xe52aa8?, 0xc0013ec390}, 0x1e, 0xc00183ca80, 0xc000088960, {0xc00013e000, 0x5a0, 0x700})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc00183d400)
	/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