Before glib 2.29.6, g_datalist_get_data was a macro of the form

#define   g_datalist_get_data(dl, k)            \
 (g_datalist_id_get_data ((dl), g_quark_try_string (k)))

At which point it was converted into a regular function in 1cceda4 (released in 2.29.6).

Whenever g_datalist_get_data was called with a non-existant key, g_quark_try_string would return 0 and the function would behave as expected.

In 06e3a1d7 (released in 2.33.12) the code of g_datalist_id_get_data was changed to use a lockless mechanism, however a new assertion was added that key_id should be != 0. The function previously silently returned NULL when used with key_id == 0.

The end result is that code compiled against glib < 2.29.6 but linked against glib > 2.33.12 will show an assertion fail whenever g_datalist_get_data is called with a string that doesn't have a quark.