This was because it caused `SIGSEGV` when `mruby -v` displayed an unnamed variable.

```console
% bin/mruby -ve 'call { |(a, b)| }'

  ...SNIP...

irep 0x8007d0050 nregs=3 nlocals=1 pools=0 syms=1 reps=1 iseq=12
file: -e
    1 000 OP_LOADSELF   R1
    1 002 OP_BLOCK      R2      I(0:0x8007d00a0)
    1 005 OP_SENDB      R1      :call   0
    1 009 OP_RETURN     R1
    1 011 OP_STOP

irep 0x8007d00a0 nregs=6 nlocals=5 pools=0 syms=0 reps=0 iseq=29
local variable names:
zsh: segmentation fault (core dumped)  bin/mruby -ve 'call { |(a, b)| }'
```