After looking a bit closer at your code. I see what's happening. If you change it as follows, the error shouldn't occur:

str3 = ""
IF NOT EMPTY(This.Value)
  str3 = "UPPER(Client.Surname) LIKE" + CHR(34) +;
  ALLTRIM(UPPER(This.Value)) + "%" + CHR(34)
ENDIF

This will save you the trouble of having to screen this character out, and allow for a smaller subset to be returned.

hth,