API- Call

I am trying to get Lead Changes via the API but with a restricted Attribute list. I’ve put together the following code but it fails when trying to access the attribute list:
 
        ParamsGetMultipleLeads leadRequest = new ParamsGetMultipleLeads();
 
        string[] attr = { "Extended_SF_18_UC_ID__c, Aconex_Key__c, Last_Updated__c" };
        leadRequest.includeAttributes = attr;
 
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                ResultGetMultipleLeads resultGetLead = leadResponse.result;
                LeadRecord[] leadRecs = resultGetLead.leadRecordList;
 
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                foreach (LeadRecord leadRecord in leadRecs)
                {
 
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                    foreach (SC_097ee0df02084e5aa161064b6c4ef557.csproj.WebReference.Attribute attrib in leadRecord.leadAttributeList)
                    {
 
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But it fails when accessing the leadAttributeList.
 
If I do not apply the filter the code works perfectly.
 
Can you check with Marketo that I am setting the includAttrutes correctly and that I can still use leadAttributeList to obtain the values of the Attributes requested.