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

goroutine 13 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000782380?}, {0xe56770?, 0xc0007824e0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000782360?}, {0xe56770?, 0xc0007824c0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000782340?}, {0xe56770?, 0xc0007824a0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000782320?}, {0xe56770?, 0xc000782480?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc000782300?}, {0xe56770?, 0xc000782460?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc000782420, 0xc00b912f00, {0xe56770, 0xc0007822e0?}, {0xc00b912eb8, {0x1db72e0, 0x4, 0x4}, 0xc00b933380, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x418408?, 0xc0002ee9e0?, 0xc000e6bdc8?, {0xe56770, 0xc0007822e0}, {0xc00b912eb8, {0x1db72e0, 0x4, 0x4}, 0xc00b933380, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc000e6be48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc00b93e9c0, {0xe52aa8?, 0xc001562390}, 0x1e, 0xc001142000, 0xc000113cb0, {0xc001592000, 0x414, 0x500})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc001142080)
	/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