panic: replaceArg: group fields don't match: 1/4

goroutine 26 [running]:
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00084e4e0?}, {0xe56770?, 0xc00084e9a0?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:377 +0x425
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00084e4c0?}, {0xe56770?, 0xc00084e980?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00084e4a0?}, {0xe56770?, 0xc00084e960?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00084e480?}, {0xe56770?, 0xc00084e940?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.replaceArg({0xe56770?, 0xc00084e460?}, {0xe56770?, 0xc00084e920?})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/prog.go:382 +0x345
github.com/google/syzkaller/prog.(*StructType).mutate(0x16d44a0, 0xc00084e580, 0xc007244ff0, {0xe56770, 0xc00084e440?}, {0xc007244e68, {0x1db72e0, 0x4, 0x4}, 0xc00736c600, ...})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:490 +0x185
github.com/google/syzkaller/prog.(*Target).mutateArg(0x0?, 0x7f62543a5108?, 0x10?, {0xe56770, 0xc00084e440}, {0xc007244e68, {0x1db72e0, 0x4, 0x4}, 0xc00736c600, ...}, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:245 +0xe2
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc006907e48)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:196 +0x245
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc0072e9f00, {0xe52aa8?, 0xc001466810}, 0x1e, 0xc00079a000, 0xc00055d560, {0xc000c6e000, 0x40c, 0x500})
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:51 +0x2ba
main.(*Proc).loop(0xc00079a240)
	/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