Index from a local variable querying data from the data table

Hello everyone, 

I have a local variable that query data from data table.The data looks like below: 

local!cases: {

(clientId: 234,group: 55),

(clientId: 600, group: 66),

(clientId:234, group: 77),

(clientId: 600, group: 88),

(clientId: 234, group: 99)

}

I want to get the group name of only client number 600. Below is my code: 

index(local!cases.group,wherecontains(600,tointeger(local!cases.clientId)),null),

And got this error:  Invalid index: Cannot index property 'group' of type Text into type List of Number (Integer)

Any one got an idea what the reason is? Thanks a lot!

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