Map without return panics

[odino]

⧐  str = ""
⧐   ["a"].map(f(x){ str = str + x })
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x4a7f03]

goroutine 1 [running]:
abs/object.(*Array).Inspect(0xc4202421a0, 0xc4202420e0, 0xc4200103d0)
	/go/src/abs/object/object.go:152 +0xc3
abs/repl.Run(0xc420244000, 0x21, 0x101)
	/go/src/abs/repl/repl.go:109 +0x109
abs/repl.executor(0xc420244000, 0x21)
	/go/src/abs/repl/repl.go:90 +0x7c
github.com/c-bata/go-prompt.(*Prompt).Run(0xc4200c8000)
	/go/src/github.com/c-bata/go-prompt/prompt.go:73 +0x7b5
abs/repl.Start(0x566a20, 0xc42000e010, 0x566a40, 0xc42000e018)
	/go/src/abs/repl/repl.go:60 +0xec
main.main()
	/go/src/abs/main.go:25 +0x11f
exit status 2
Makefile:11: recipe for target 'repl' failed
make: *** [repl] Error 1