Search with Lookup Field

Can someone help me fix the error?

Error: In Criteria left expression is of type BIGINT and right expression is of type STRING and the operator == is not valid

Code:

htmlpage Search(searchid)
<%{ name = Personen[ID == input.searchid];%>

<%=name.Name%>
<%=name.Vorname%>

<%}%>

I'm still a beginner, Thanks for Help