Error: RetrieveMultiple - The ConditonOperator.Equal requires 1 value/s, not 0. Parameter Name: LastName

<s:Body>
    <RetrieveMultiple xmlns="schemas.microsoft.com/.../Services">
        <query i:type="b:QueryExpression" xmlns:b="schemas.microsoft.com/.../Contracts" xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns:c="schemas.microsoft.com/.../Arrays">
            <b:ColumnSet>
                <b:AllColumns>false</b:AllColumns>
                <b:Columns xmlns:c="schemas.microsoft.com/.../Arrays">
                    <c:string>firstname</c:string>
                    <c:string>lastname</c:string>
                </b:Columns>
            </b:ColumnSet>
            <b:Criteria>
                <b:Conditions>
                    <b:ConditionExpression>
                        <b:AttributeName>lastname</b:AttributeName>
                        <b:Operator>Equal</b:Operator>
                        <b:Values xmlns:b="schemas.microsoft.com/.../Arrays">
                            <c:anyType i:type="c:string" xmlns:c="www.w3.org/.../XMLSchema">Something</c:anyType>
                        </b:Values>
                    </b:ConditionExpression>
                </b:Conditions>
            </b:Criteria>
            <b:EntityName>contact</b:EntityName>
            <b:Distinct>false</b:Distinct>
            <b:LinkEntities />
            <b:Orders />
            <b:TopCount>5</b:TopCount>
            <b:PageInfo>
                <b:Count>0</b:Count>
                <b:PageNumber>0</b:PageNumber>
                <b:PagingCookie i:nil='true'/>
                <b:ReturnTotalRecordCount>false</b:ReturnTotalRecordCount>
            </b:PageInfo>
        </query>
    </RetrieveMultiple>
</s:Body>