Custom Lists

I have created a custom list and assigned it to a Custom CRM field and applied it to Case records.  Using the web services I am trying to set this value programatically by doing this:  (some items omitted for brevity).

CustomFieldRef cfr = new StringCustomFieldRef();    
cfr.value = <the value of the field>;
cfr.internalId = <the internal id of the list item>;

then

CustomFieldRef[] cfrAry = new CustomFieldRef[1];
cfrAry[0] = cfr;

newRecord.customFieldList = cfrAry;

I know the internal Id is correct, but I'm getting this error:
Code=INVALID_CSTM_FIELD_DATA_TYP] The customfield [CUSTEVENTCUSTTZ] reference object does not match its data type.