Tabulation not working in CSS part of vue-mode

I get an error each time I try to use in the CSS part of a vue file.
Error : `vue-mmm-indent-line-narrowed: Symbol’s function definition is void: nil`

```
mmm-mode-0.5.7
vue-mode-20180516.312
```

Thank you

Reproduce the error by pressing TAB in the CSS part of a Vue file using mmm-mode 0.5.7 and vue-mode 20180516.312. Trace the vue-mmm-indent-line-narrowed error to determine why indentation fails; done means TAB works in that CSS section without the reported error.

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