gtk_css_value_inherit_free: code should not be reached

Question

I making one plugin with Python for Rhythmbox.

I get an randomly error when start the plugin. Then of some seconds I reset Rhythmbox and the plugin run ok.

What will the cases that probably originy the error?

Error:

gtkcssinheritvalue.c:33:gtk_css_value_inherit_free: code should not be reached

gtkcssinheritvalue.c:

29  static void
30  gtk_css_value_inherit_free (GtkCssValue *value)
31  {
32    /* Can only happen if the unique value gets unreffed too often */
33    g_assert_not_reached ();
34  }

https://github.com/GNOME/gtk/blob/gtk-3-6/gtk/gtkcssinheritvalue.c

All suggestion is welcome. Thanks.