proto_tree_add_bits_item(), bit limit?

I am trying to use proto_tree_add_bits_item() to select all the bits that are used in a 7-bit ASCII message of arbitrary length. I convert this 7-bit ASCII to 8-bit and then print out a string, but I would like Wireshark to select the bits that this string was derived from when the "Packet Bytes" window is in "Bits View" instead of the standard "Hex view". I have found that it will highlight the individual bits up to 7*3=21, and then it starts highlighting whole bytes. What's up?

ti = proto_tree_add_bits_item(cftext_sub_tree3, hf_cftext_textmess, tvb, 1918, 7*num_char, FALSE);
/* I am not interested in printing the bits here,

that's why I am using proto_item_set_text() instead of
proto_item_append_text().
*/
proto_item_set_text(ti, "Text Message: %s", ascii_message);

Any ideas/answers are greatly appreciated!