Liquibase "modifyDataType" tag always makes the column nullable

When I do-

<modifyDataType 
    tableName="EMCostUnit" 
    columnName="GUID" 
    newDataType="VARCHAR(50)"/>

In liquibase, it always resets the nullable constraints. Is it possible to add some parameter so it didn't remove the constraints, because now after each modifyDataType, I need additional changeSet to add back not null constraints.