panic: replaceArg: group fields don't match: 4/0

goroutine 24 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abcc80?}, {0xe56770?, 0xc000ac0860?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abcca0?}, {0xe56770?, 0xc000ac0800?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abccc0?}, {0xe56770?, 0xc000ac07e0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abcd00?}, {0xe56770?, 0xc000ac0580?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abcd20?}, {0xe56770?, 0xc000ac0560?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abcd40?}, {0xe56770?, 0xc000ac0540?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abcd60?}, {0xe56770?, 0xc000ac0520?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000abcd80?}, {0xe56770?, 0xc000ac0500?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc000abc700, 0xc00c1e1f90, {0xe56770, 0xc000abcda0?}, {0xc00c1e1db8, {0x1db72e0, 0x4, 0x4}, 0xc00c29cea0, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x0?, 0x7fcd2dc2e108?, 0x40?, {0xe56770, 0xc000abcda0}, {0xc00c1e1db8, {0x1db72e0, 0x4, 0x4}, 0xc00c29cea0, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc000a2fe48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc00c298ec0, {0xe52aa8?, 0xc00105a060}, 0x1e, 0xc001674000, 0xc0000ed8c0, {0xc0004b6000, 0x502, 0x700})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc0016740c0)
	/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