panic: replaceArg: group fields don't match: 0/2

goroutine 35 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4d40?}, {0xe56770?, 0xc0008e4ea0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4d20?}, {0xe56770?, 0xc0008e4e80?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4d00?}, {0xe56770?, 0xc0008e4e60?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4ce0?}, {0xe56770?, 0xc0008e4e40?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4cc0?}, {0xe56770?, 0xc0008e4e20?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4ca0?}, {0xe56770?, 0xc0008e4e00?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4c80?}, {0xe56770?, 0xc0008e4de0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0008e4c60?}, {0xe56770?, 0xc0008e4dc0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc0008e4d80, 0xc00bc015e0, {0xe56770, 0xc0008e4c40?}, {0xc00bc01598, {0x1db72e0, 0x4, 0x4}, 0xc00bc5d050, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x418408?, 0xc000394500?, 0xc0009b1dc8?, {0xe56770, 0xc0008e4c40}, {0xc00bc01598, {0x1db72e0, 0x4, 0x4}, 0xc00bc5d050, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc0009b1e48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc00bc33e40, {0xe52aa8?, 0xc0012f4690}, 0x1e, 0xc0015a20c0, 0xc000096f00, {0xc0014d2000, 0x41f, 0x500})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc0015a2200)
	/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