private mib definition problem

i define private mib files.

1.I want a 64 bit INTEGER as a table's index. Then I use SYNTAX
"Counter64" as this table's index

2.complie this mib file using tool "adventnet simulation toolkit". This
tool give me following message:

 

   ERR-162              The objects having the syntax Counter32 or
Counter64,

                   must not be specified in an INDEX clause.

 

My question is :why I cann't using SYNTAX "Counter64" as a table's
index, if cann't using it ,which syntax I can use? 

 

thanks

  

Chunmei