When creating a new dataset the fields default to any and thus cannot be used in reports. This might just be for strings. I did use a number field in my select statement and it did default to Integer. When the dataset or a single field is dragged to the report, end error occurs that says that fields cannot be of type "any".

The report xml can be edited to set the correct type but if the dataset is edited, they seem to revert to any. This may only happen if the query is edited. The type cannot be changed in the dataset editor though.