Document without title causes unhelpful error in template

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I created a document without a title, which is perfectly valid, but when I try to use that document in a template, I get the following error:

Error in template: Undef did not pass type constraint "Str" (in $self->{"title"}) at (eval 652) line 68
    "Str" is a subtype of "Value"
    "Value" is a subtype of "Defined"
    Undef did not pass type constraint "Defined" (in $self->{"title"})
    "Defined" is defined as: (defined($_))

Nowhere in the error message tells me where the problem is, what document I'm trying to write, or how I could possibly fix this.

Running under -vv verboseness gave me nothing useful:

[Sun Jan 24 01:57:49 2016] [debug] Write file: /blog/2015/04/26/chicken-broccoli-alfredo/making-sauce-2.jpg
[Sun Jan 24 01:57:49 2016] [debug] Write file: /blog/2015/04/12/cheddar-cheese-sauce/adding-cheese-3.jpg
[Sun Jan 24 01:57:49 2016] [debug] Write file: /blog/2015/04/05/banana-bread/add-sugar-2.jpg
[Sun Jan 24 01:57:49 2016] [debug] Write file: /blog/2015/04/05/the-mac-and-cheese-experiment---3---pre-made-sauces/add-velveeta-cheese-1.jpg
[Sun Jan 24 01:57:49 2016] [debug] Write file: /blog/2015/09/05/baked-sweet-and-sour-chicken/index.html
Error in template: Undef did not pass type constraint "Str" (in $self->{"title"}) at (eval 652) line 68
    "Str" is a subtype of "Value"
    "Value" is a subtype of "Defined"
    Undef did not pass type constraint "Defined" (in $self->{"title"})
    "Defined" is defined as: (defined($_))

Running under Carp::Always gives me more to go on:

Error in template: Undef did not pass type constraint "Str" (in $self->{"title"}) at /Users/doug/perl5/perlbrew/perls/perl-5.20.0/lib/site_perl/5.20.0/Statocles/Template.pm line 133.
        Statocles::Template::render(Statocles::Template=HASH(0x7ff5c3ee1b08), "content", "<h1>indiepalate.com</h1>\x{a}\x{a}<p>This is the source and files for"..., "app", Statocles::App::Basic=HASH(0x7ff5c6394b50), "doc", Statocles::Document=HASH(0x7ff5c3f9cdd0), "site", Statocles::Site=HASH(0x7ff5c3b94b30), ...) called at /Users/doug/perl5/perlbrew/perls/perl-5.20.0/lib/site_perl/5.20.0/Statocles/Page.pm line 177
        Statocles::Page::render(Statocles::Page::Document=HASH(0x7ff5c3f17510), "site", Statocles::Site=HASH(0x7ff5c3b94b30)) called at /Users/doug/perl5/perlbrew/perls/perl-5.20.0/lib/site_perl/5.20.0/Statocles/Site.pm line 291
        Statocles::Site::build(Statocles::Site=HASH(0x7ff5c3b94b30)) called at /Users/doug/perl5/perlbrew/perls/perl-5.20.0/lib/site_perl/5.20.0/Statocles/Site.pm line 359
        Statocles::Site::deploy(Statocles::Site=HASH(0x7ff5c3b94b30)) called at /Users/doug/perl5/perlbrew/perls/perl-5.20.0/lib/site_perl/5.20.0/Statocles/Command.pm line 122
        Statocles::Command::main("Statocles::Command", "-vv", "deploy") called at /Users/doug/perl5/perlbrew/perls/perl-5.20.0/bin/statocles line 16

When there's an error in the template, we need to give enough information to solve the problem. We should also add "Rendering page: " as debug-level (-vv) output to try to help with these problems.