* Command is probably a wrong term for this. You are referring to a value /
property so I guess what you posted rstExt.Fields(0).Value is only a portion
of a full VBA statement. Generally, it is quite OK to refer to a Null value
but it will be incorrect in other contexts, e.g. assigning the Null value to
a String / Long / etc ... variable.
I guess you need to post the context you are using rstExt.Fields(0).Value,
i.e. more related code.
* You should also post the error number / error message so that potential
respondents have more info. to go on.
* "Empty" could meant Null or zero-length String (2 different things
altogether) so you need to post the data type of Fields(0) also.
* Did you check for empty Recordset (zero row returned) before you referred
to the Recordset's Field value?