Converting v3 extension to v4

I’ve attempted to convert a simple extension (middleman-title) from v3 to v4. However, somehow, I can’t get my views to recognize the helper I’ve registered.

I keep getting NameError: undefined local variable or method 'page_title'.

If anyone was kind enough to take a look, that’d be really awesome: https://github.com/cmer/middleman-title/tree/middleman_4_compatibility

It must be something really silly… Thanks in advance!