not well-formed (invalid token) at line 15, column 51, byte 720 when trying to parse XMLRPC call

I'm running WP 4.7.4. The site is a default site with no plugins activated other than the default plugins. I'm using the WordPress::XMLRPC Perl module to generate a getPost XMLRPC call for post #1 like so:

my $obj = WordPress::XMLRPC-new({username = 'admin', password = 'xxxxxx', proxy = 'http://my.domain.com/'});

my $post = $obj-getPost(1);

The full error is this:

not well-formed (invalid token) at line 15, column 51, byte 720 at /usr/local/lib/x86_64-linux-gnu/perl/5.20.2/XML/Parser.pm line 187. !DOCTYPE html> html lang="en-US" class="no-js"> head> ...snip...> /body> /html> at /usr/local/share/perl/5.20.2/WordPress/XMLRPC.pm line 643.

Here is line 643 from WordPress::XMLRPC:

    my $call = $self-server-call(
            'metaWeblog.getPost',
            $post_id,
            $user_login,
            $user_pass,
    );

Here is line 187 from XML::Parser in an eval statement:

eval {
  $result = $expat-parse($arg);
}; 

Line 15 of the output is this:

https://fonts.gstatic.com' crossorigin rel='preconnect' />

When I run the output through this XML validator, I also get an error:

This page contains the following errors: error on line 15 at column 52: Specification mandate value for attribute crossorigin