SM30 is not updating particular type of value

Hi Friends

I created SM30 for custom table.its working fine in all the cases except one.

I have one field with type char2. I have dinfferent types of value in table like "02", "01" "E" and "S".

if it is "E" or "S" its not allowing me to modify into "02" or "01".

But I can able to modify "02" to "01" / "01 to "02".

I am not sure what the issue here, I checked there is no check table/Value table for this field.

I regenerated maintenance table and checked..still I am facing the same issue.

Helpful answer will be apreciated.

Thanks

Gowirshankar