Error Log : Theme File Generation Failed

For every new site I always see this error in Error Log:

Traceback (most recent call last):
  File "/home/user/frappe-bench/apps/frappe/frappe/website/doctype/website_theme/", line 115, in generate_theme_files_if_not_exist
  File "/home/user/frappe-bench/apps/frappe/frappe/website/doctype/website_theme/", line 86, in generate_theme_if_not_exist
  File "/home/user/frappe-bench/apps/frappe/frappe/website/doctype/website_theme/", line 73, in generate_bootstrap_theme
    frappe.throw('<div style="font-family: monospace;">{stderr}</div>'.format(stderr=stderr))
  File "/home/user/frappe-bench/apps/frappe/frappe/", line 376, in throw
    msgprint(msg, raise_exception=exc, title=title, indicator='red', is_minimizable=is_minimizable)
  File "/home/user/frappe-bench/apps/frappe/frappe/", line 355, in msgprint
  File "/home/user/frappe-bench/apps/frappe/frappe/", line 315, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.ValidationError: <div style="font-family: monospace;">undefined<br></div>

Is there anything I did wrong?
But I think this is a bug because website theme is empty until user create one.
If someone can confirm this is a bug I will post an issue in github.
Thank you.

I am having the same error.
And couldn’t find anything on it.