crm 2011 ribbon: disable system button - dynamics-crm-2011

Question

I have a problem with disabling ribbon button via javascript custom rule.
The action for that I want to add "enable rule" is called Mscrm.AddExistingRecordFromSubGridAssociated -> which is displayed when subgrid gets focus.
I use the ribbon workbench solution and I almost sure I did everything fine, but it doesn't work.
javascript:
utils.ribbon.isAddExistingDisabled = function(grid) {
return false; //just for test I wrote always false, but it's enabled anyway :/
}
CommandDef:
<CommandDefinition Id="Mscrm.AddExistingRecordFromSubGridAssociated">
<EnableRules>
<EnableRule Id="Mscrm.AppendToPrimary" />
<EnableRule Id="Mscrm.EntityFormIsEnabled" />
<EnableRule Id="my.incident.addMainCause.EnableRule" />
</EnableRules>
<DisplayRules>
<DisplayRule Id="Mscrm.AddExisting" />
<DisplayRule Id="Mscrm.ShowForManyToManyGrids" />
<DisplayRule Id="Mscrm.AppendToPrimary" />
<DisplayRule Id="Mscrm.AppendSelected" />
</DisplayRules>
<Actions>
<JavaScriptFunction FunctionName="Mscrm.GridRibbonActions.addExistingFromSubGridAssociated" Library="/_static/_common/scripts/RibbonActions.js">
<CrmParameter Value="SelectedEntityTypeCode" />
<CrmParameter Value="SelectedControl" />
</JavaScriptFunction>
</Actions>
</CommandDefinition>
Enable rule def:
<EnableRule Id="my.incident.addMainCause.EnableRule">
<CustomRule FunctionName="utils.ribbon.isAddExistingDisabled" Library="$webresource:my_js/incident/common.js" InvertResult="false">
<CrmParameter Value="SelectedControl" />
</CustomRule>
</EnableRule>