wrong type argument stringp, nil

Hi, I think I found a bub in XEmacs 21.1.10-5 (I installed it with the debian package). Actually it's not very easy to explain how the probleme appeared. I was working on a file a wanted to open an other file in a buffer when I got the warning i put in the subject of this email. I tried to open an other and got the same trouble. I tried to load an other XEmacs open the file I was first working on and then the second file in a new buffer but didn't get any warning and I was that time able to open the second file. I hope this report is not to confusing and may be helpful. If you have any clue of details you may need, feel free to reply me. JM