unable to delete branch: "Internal Error"

[brainchild0]

The application is failing to delete a branch.

(Note that in this case the name of the branch is master. It was created by the application to initialize the repository, but is not needed.)

The web interface prints a message "Internal Error", and the log shows the following:

[Wed May 27 20:47:33 2020] [error] /gitprep.cgi/user/repo/branches?op=delete: Branch deleting failed. Can't delete branch master at template /branches.html.ep line 40.
95:     # Icky backwards compatibility wrapper. :-(
96:     local @CARP_NOT = $cgc ? $cgc->() : caller();
97:     shortmess_heavy(@_);
98: }
99: 
100: sub croak   { die shortmess @_ }
101: sub confess { die longmess @_ }
102: sub carp    { warn shortmess @_ }
103: sub cluck   { warn longmess @_ }
104: 
105: BEGIN {