Strange Error - CS0012: The type x is defined in an assembly that is not referenced

<uc1:usercontrol id="ucusercontrol " runat="server" myPublicUserControlProperty='<%#Eval("CollectionOfX") %>'/>

<assemblies>
        <add assembly="MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=[MyPublicKeyToken]"/>
</assemblies>