User Control 'tag prefix' error

I keep getting an error saying "This control cannot be displayed because it's TagPrefix is not registered in this Web Form" but I've tried defining the tagprefix both on the top of page and then in the web config pages section (????).
Page 
<trt: Antivirals ID="antivirals1" runat="server"/>
Web Config
<add tagPrefix="trt" tagName="Antivirals" src="~/Controls/Antivirals.ascx"/>



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