unpack error in fsa_tree_file.php

Occasionally when I do a product search I get the following error Warning: unpack(): Type V: not enough input, need 4, have 0 in /home/.../system/library/phpmorphy/src/fsa/access/fsa_tree_file.php line 42 and the same for line 88. Can anybody shed any light on why this might be? It seems random i.e. sometimes I will search for "mal" or "malay" and receive the error and other times not and searching for "dak" makes it throw the error but "dakota" will show the product of that name. The site is at http://www.mosaicsandtiles4you.com/new/.
I'm using opencart 1.5.6.4_rc.
Thanjs

If I turn off error reporting the message changes to
Error occured while text processing: Infinite recursion possible
and I get an entry in the error log saying
<br />
<b>Fatal error</b>: Allowed memory size of 67108864 bytes exhausted (tried to allocate 23310336 bytes) in <b>/home/..../admin/view/template/tool/error_log.tpl</b> on line <b>17</b><br />

Does anyone have ideas please?

Problem solved - Filezilla had reset itself to upload in ASCII mode rather than binary when I uploaded a search extension. I've reuploaded the files in binary mode and it is now working.