Valid API Blueprint generates "Error: Error parsing input: unknown type of mson value"

[input.txt](https://github.com/danielgtaylor/aglio/files/75820/input.txt)

I've created a relatively large API Blueprint (approx. 160 Kb, 4359 lines of code) which is displaying in Apiary.io's editor as a "Valid document". The Apiary.io documentation is generating as intended.

However, when I attempt to run aglio using the command:
`aglio --verbose -i input.apib -o output.html`

or

`aglio --verbose -i input.apib -o output.html BENCHMARK=1 NOCACHE=1`

I receive an error that reads:

> Loading theme default
> Error: Error parsing input: unknown type of mson value

As the Apiary.io Editor and Documentation seems to interpret the blueprint with no problems, I suspect this is some sort of a parsing error in Aglio.

When I trimmed the API Blueprint to just three endpoints and their supporting `# Data Structures` sections (11 Kb, 310 lines) I still get the same error.

Is this a bug that anyone else has run into in Aglio or the parsers it uses? If not, then would it be possible to somehow get a more verbose error message so that I can spot where the problem spot in my blueprint is?

Edited to include the trimmed API Blueprint as "input.txt" in the attached document.