How do I explicitly insert nulls into a parametized query?

I'm using Delphi 7 and Firebird 1.5.

I have a query that I create at runtime where some of the values might be null. I can't work out how to get Firebird to accept explicit nulls for values that I need to leave as null. At this stage I'm building the SQL so that I don't include parameters that are null but this is tedious and error-prone.

var  Qry: TSQLQuery;begin  SetConnection(Query); // sets the TSQLConnection property to a live database connection  Query.SQL.Text := 'INSERT INTO SomeTable (ThisColumn) VALUES (:ThisValue)';  Query.ParamByName('ThisValue').IsNull := true; // read only, true by default  Query.ParamByName('ThisValue').Clear; // does not fix the problem  Query.ParamByName('ThisValue').IsNull = true; // still true  Query.ParamByName('ThisValue').Bound := true; // does not fix the problem  Query.ExecSQL;

Currently an EDatabaseError "No value for parameter 'ThisValue'"' is raised in DB.pas so I suspect this is by design rather than a firebird problem.

Can I set parameters to NULL? If so, how?

(edit: sorry for not being explicit about trying .Clear before. I left it out in favour of mentioning IsNull. Have added declaration and more code)

Sorry, one more thing: there is no "NOT NULL" constraint on the table. I don't think it's getting that far, but thought I should say.

Complete console app that displays the problem at my end:

program InsertNull;{$APPTYPE CONSOLE}uses  DB,  SQLExpr,  Variants,  SysUtils;var  SQLConnection1: TSQLConnection;  Query: TSQLQuery;begin  SQLConnection1 := TSQLConnection.Create(nil);  with SQLConnection1 do  begin    Name := 'SQLConnection1';    DriverName := 'Interbase';    GetDriverFunc := 'getSQLDriverINTERBASE';    LibraryName := 'dbexpint.dll';    LoginPrompt := False;    Params.clear;    Params.Add('Database=D:\Database\ZMDDEV12\clinplus');    Params.Add('RoleName=RoleName');    //REDACTED Params.Add('User_Name=');    //REDACTED Params.Add('Password=');    Params.Add('ServerCharSet=');    Params.Add('SQLDialect=1');    Params.Add('BlobSize=-1');    Params.Add('CommitRetain=False');    Params.Add('WaitOnLocks=True');    Params.Add('ErrorResourceFile=');    Params.Add('LocaleCode=0000');    Params.Add('Interbase TransIsolation=ReadCommited');    Params.Add('Trim Char=False');    VendorLib := 'gds32.dll';    Connected := True;  end;  SQLConnection1.Connected;  Query := TSQLQuery.Create(nil);  Query.SQLConnection := SQLConnection1;  Query.Sql.Text := 'INSERT INTO crs_edocument (EDOC_ID, LINKAGE_TYPE) VALUES (999327, :ThisValue)';  //Query.ParamByName('ThisValue').IsNull := true; // read only, true by default//  Query.ParamByName('ThisValue').Value := NULL;  Query.ParamByName('ThisValue').clear; // does not fix the problem  Query.ParamByName('ThisValue').Bound := True; // does not fix the problem//  Query.ParamByName('ThisValue').IsNull; // still true  Query.ExecSQL;end.