Problem loading different tables

[bernie61]

I have an application that shows a table in a form and allows the user to select another different table to be shown in the same form.

It calls a routine passing the table number (as string variable "t") to load and show it.

The first time I call this procedure dbf1 is not active, then it simply loads the first table.

  if dbf1.active then begin 
    dbf1.PackTable;
    dbf1.Close;
    dbf1.active:=false;
  end;
  dbf1.TableName:='table' + t + '.dbf';
  dbf1.active:=true;
  dbf1.IndexDefs.Clear;
  dbf1.AddIndex('GINDEX','STR(GROUP,5,0)+STR(SUBGROUP,5,0)', [ixCaseInsensitive]);

The problem is that it works well only one, two or three times, but in some moment, when I switch tables again, I get an exception:

DBFError
Exclusive access is required for this operation.

I get this exception either when packing the table or when adding the index.

But the table has exclusive:=true and there only one instance of the application.

I use Lazarus 1.6.4, 64 bits on Windows 7.

What I'm doing wrong?