Others have said that DLookups in a query don't make a whole lot of sense (partly because they're inefficient), and I tend to agree with them. If you can't include the needed table in the query, there might be something wrong with the relationships. If using it anyway, you should make sure the value it returns is unique else it will return the first occurrence and that might be incorrect. A query would reveal this since your record count would be more than you expect. If you intend to use a query with DLookup as a source for an append or update query, you'd also want to ensure the target field will accept null in case the lookup returns null (if it is to be appended/updated), or be sure it never will.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.