Unigrid filter parameters not calling SetValue

I'm following the documentation here:

https://docs.kentico.com/display/K82/Reference+-+UniGrid+definition#Reference-UniGriddefinition-Settingparametersforcustomcolumnfilters

This is from my UniGrid defition:

<column source="MediaTypeID" caption="Type" externalsourcename="#transform: wvo.mediatype.mediatypelabel" wrap="false">
  <filter type="custom" path="~/Filters/MediaTypeFilter.ascx" format="[{0}] IN (SELECT MediaTypeID FROM WVO_MediaType WHERE MediaTypeLabel {1} {2})">
    <Parameters>
      <filterparameter name="Column" value="MediaTypeID" />
    </Parameters>
  </filter>    
</column>

My filter (MediaTypeFilter.ascx) implements ISimpleDataContainer per the instructions:

/// <summary>
/// Gets or sets the value of the column.
/// </summary>
/// <param name="columnName">Name of the column.</param>
public object this[string columnName]
{
    get
    {
        return GetValue(columnName);
    }
    set
    {
        SetValue(columnName, value);
    }
}

/// <summary>
/// Returns value of column.
/// </summary>
/// <param name="columnName">Column name</param>
public object GetValue(string columnName)
{
    switch (columnName.ToLower())
    {
        case "column":
            return this.Column;
    }
    return null;
}

/// <summary>
/// Sets value of column.
/// </summary>
/// <param name="columnName">Column name</param>
/// <param name="value">Column value</param>
public bool SetValue(string columnName, object value)
{
    EventLogProvider.LogInformation("SetValue", "SetValue", "AM I HERE? ::: " + columnName + " ::: " + value);
    switch (columnName.ToLower())
    {
        case "column":
            this.Column = ValidationHelper.GetString(value, "");
            return true;
    }
    return false;
}

... but that SetValue method never gets called. What am I missing?

Thanks!