Wrong type argument: char-or-string-p

[Richard Burton]

 I'm using Rmail in emacs.  To reply to a message, I enter "r."  To quote
the original message, I enter c-c c-y.  According to info, the original
message should be quoted with the value of mail-yank-prefix.
 
 So I try:
 
 (set mail-yank-prefix '>)
 
 But it has no effect on the behavior of c-c c-y, except that it generates
the error message "Wrong type argument: char-or-string-p."
 
 What am I doing wrong?
 
 Thanks,
 
rb