Language is either not supported or WIP... c++mode

[marlinstrub]

This package looks very promising. When I try ts-docstr-at-point I get the error Language is either not supported or WIP... c++mode.

I load tree-sitter and ts-docstr as follows:

(use-package tree-sitter
  :straight (tree-sitter :host github :repo "emacs-tree-sitter/elisp-tree-sitter"))

(use-package tree-sitter-langs
  :straight (tree-sitter-langs :host github :repo "emacs-tree-sitter/tree-sitter-langs"))

(use-package msgu
  :straight (msgu :host github :repo "jcs-elpa/msgu"))

(use-package ts-docstr 
  :straight (ts-docstr :host github :repo "emacs-vs/ts-docstr"))

I then visit a cpp file and call M-x ts-docstr-at-point on a function.

Am I doing this wrong?