Assertion failed when trying to validate an AppStream appdata file

[AsciiWolf]

I am getting the following failed assertion message when trying to validate this file:

$ appstream-util validate-relax gqrx.appdata.xml
gqrx.appdata.xml:
(appstream-util:2666): GLib-CRITICAL **: g_strstr_len: assertion 'haystack != NULL' failed
(appstream-util:2666): GLib-CRITICAL **: g_strstr_len: assertion 'haystack != NULL' failed
(appstream-util:2666): GLib-CRITICAL **: g_strstr_len: assertion 'haystack != NULL' failed
(appstream-util:2666): GLib-CRITICAL **: g_strstr_len: assertion 'haystack != NULL' failed
(appstream-util:2666): GLib-CRITICAL **: g_strstr_len: assertion 'haystack != NULL' failed
(appstream-util:2666): GLib-CRITICAL **: g_strstr_len: assertion 'haystack != NULL' failed
OK

$ appstream-util --version
Version: 0.6.6

[AsciiWolf]

Oops, I see that the appstream-util is not a part of this repository.