RichTextBox: problems inserting (c), TM and (R) symbols

[yuraukar]

I am trying to insert special characters into text of a
rich text edit box. The user should be able to press a
toolbarbutton to insert a specific character.
I have looked up RTF codes for (c) and (R) etc. and tried
the following code:

richTextBox1.SelectedRtf = @"{\rtf1\ansi \'99}";

This properly inserts a "TM" symbol in the text.

However, to my surprise, the box seems "screwed" after
this. I can edit the text that was there before the
insert, but not delete or overwrite the TM symbol. The
same applies to (c) and other symbols.

I also tried

richTextBox1.Rtf = @"{\rtf1 Some Symbol\'99}";

which results in a RTF box that cannot be edited any
more, but

richTextBox1.Rtf = @"{\rtf1 Some Text}";
works fine.

What's wrong here??