Nulls in delphi fields

I do the following in sap thru SQLStudio:

update MyTable set MyField = null

MyField is FIXED(15,4)

and, I do the following in delphi:

query1.open.
query1.edit;
query1['MyField'] := query1['MyField'];
query1.post;

and system post the message:

[SAP AG][SQLOD32 DLL]Numeric value out of range.

it work fine in Oracle, Access and SQLServer, the value remains in null and
all we are happy.

The second situation is to use FLOAT instead of FIXED, but in the above
operation the value chance null by 0 and it is an non-desired behavior.

I'm using SAP DB 7.4.3.27 and the last ODBC driver.

Can somebody help me.

Thanks in advance and sorry if the problem is not for this forum.

Ricardo Solanilla