g_log_set_fatal_mask() not working for me.

[Richard Shann <richard rshann plus com>]

I am getting a warning message:

 Gtk-WARNING **: Allocating size to GtkScrollbar 0x55555647efd0 without
calling gtk_widget_get_preferred_width/height(). How does the code know
the size to allocate?

however when I call 

call g_log_set_fatal_mask ("Gtk", 1<<4)

at the start of main the warning is not converted into a fatal error.
I've tried

call g_log_set_fatal_mask ("Gtk", 0xFFFF)

as well, to no avail.

This is with Gtk version runtime: 3.22.11, compiled against: 3.22.11

Any suggestions to track down the source of the trouble gratefully
received...

Richard Shann