ERROR 1170 (42000) : BLOB/TEXT

Saturday 30 September 2006 6:05:19 am

hello friends,
i am using mysql along with php. i am using xml for creating dbase. I hv create a table with some fileds among them 1 is
<field>
<name>memo_data</name>
<type>text</type>
<comment>data for ext property data_type = memo.</comment>
<add>1.1</add>
</field>
<index>
<name>index_memo_field</name>
<fieldName>memo_data</fieldName>
<fieldName>custom_field_id</fieldName>
<add>1.6</add>
</index>
the problem is when i run this,then the error appears "ERROR 1170 (42000) at line 2717: BLOB/TEXT column 'memo_data' used in key specification without a key length".
this problem is solved when i replace text with varchar and limit the length to 255 but i want it to remain text......
can any1 help me????? the meaning that i interpret from this errror msg is that limit of text should be defined but this is not possible in mysql for type text.
Regards
abcd