I have a component that accepts a Boolean attribute. Certain combinations of inline functions generate the error message shown in the subject line. See code below for details:

<!-- this produces the error --> <c:myComponent myBooleanValue="{!LEN(myObj__c.myField__c) == 0}" .../> <!-- this produces the error --> <c:myComponent myBooleanValue="{!LEN(myObj__c.myField__c) = 0}" .../> <!-- this works! --> <c:myComponent myBooleanValue="{!NOT(LEN(myObj__c.myField__c) > 0)}" .../>

Obviously I have a workaround (the last example) but I'm curious to know why the other two do not work.