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

goroutine 27 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0009f7e40?}, {0xe56770?, 0xc00043d660?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0009f7e20?}, {0xe56770?, 0xc00043d640?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0009f7e00?}, {0xe56770?, 0xc00043d600?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0009f7de0?}, {0xe56770?, 0xc00043d540?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc0009f7dc0?}, {0xe56770?, 0xc00043d520?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc0009f7fa0, 0xc000734870, {0xe56770, 0xc0009f7da0?}, {0xc000734738, {0x1db72e0, 0x4, 0x4}, 0xc0015eb860, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x0?, 0xc00039fa20?, 0xc001ad7dc8?, {0xe56770, 0xc0009f7da0}, {0xc000734738, {0x1db72e0, 0x4, 0x4}, 0xc0015eb860, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc001ad7e48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc0016851c0, {0xe52aa8?, 0xc000d2c750}, 0x1e, 0xc000e742c0, 0xc000088f00, {0xc00098a000, 0x418, 0x500})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc000e74c40)
	/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