Gtk.CssProvider font-family works only some of the time

I have a C program with these excerpts:

// Create new window to display unfragmented packet.
  window1 = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_window_set_resizable (GTK_WINDOW (window1), TRUE);
  gtk_window_set_position (GTK_WINDOW (window1), GTK_WIN_POS_NONE);
  gtk_window_set_title (GTK_WINDOW (window1), title[0]);
  gtk_window_set_default_size (GTK_WINDOW (window1), 1000, 200);
  gtk_container_set_border_width (GTK_CONTAINER (window1), 1);

  // Create a textview object to display unfragmented packet.
  textview1 = gtk_text_view_new ();
  buffer1 = gtk_text_view_get_buffer (GTK_TEXT_VIEW (textview1));
  gtk_text_view_set_cursor_visible (GTK_TEXT_VIEW (textview1), TRUE);
  provider = gtk_css_provider_new ();
  gtk_css_provider_load_from_path (provider, "./packet.css", NULL);
  context = gtk_widget_get_style_context (textview1);
  gtk_style_context_add_provider (context, GTK_STYLE_PROVIDER (provider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);

I then put my text into nofrag_string.

And display:

  // Put the text into the buffer.
  gtk_text_buffer_set_text (buffer1, nofrag_string, -1);

  // Add scrolled window and textview to new window1 and display unfragmented packet.
  scrolled_win1 = gtk_scrolled_window_new (NULL, NULL);
  gtk_container_add (GTK_CONTAINER (scrolled_win1), textview1);
  gtk_scrolled_window_set_policy (GTK_SCROLLED_WINDOW (scrolled_win1),
                                  GTK_POLICY_AUTOMATIC, GTK_POLICY_ALWAYS);

  gtk_container_add (GTK_CONTAINER (window1), scrolled_win1);
  gtk_widget_show_all (window1);

and a CSS file packet.css:

textview {
  font-family: Courier;
}

This works, except when it tries to display "ff":

For some reason, it changes to a proportional font for "ff". What I want is a non-proportional font always so that my output looks similar to what you'd get from xxd.