"Fatal scripting error" when parsing SQLCMD query

*Similar issue seen [here](https://dba.stackexchange.com/questions/223664/sql-command-mode-unable-to-run-command).* I am trying to run the following command from SSMS running in SQLCMD mode.

:r C:\dev\test scripts\sample.sql

But I get the following error:

A fatal scripting error occurred.
Incorrect syntax was encountered while parsing :r .

I have checked the file, the line endings are appropriate for the Windows machine, and the batch separator matches the settings. What else can I try to run this?