I am trying to do trailhead Custom Metadata Types Basics, but...

In Use Custom Metadata Types in Formulas, Default Values, and Validation (salesforce.com), I create a validation rule Revenue_Validation with Error Condition Formula like 

IF( 
 (ISPICKVAL( Support_Tier__c , "Gold") 
      && 
   (  AnnualRevenue >  
   ($CustomMetadata.Support_Tier__mdt.Gold.Minimum_Spending__c * 20) 
   )
 ), TRUE, FALSE
)

But as a result, I get the message: In the validation rule named 'Revenue_Validation', we can’t find a reference to '$CustomMetadata.Support_Tier__mdt.Gold.Minimum_Spending__c'.  Where did I get it wrong?