$__timeGroup with fillnull and $__unixEpochFilter

I can’t use the $__timeGroup(dateColumn, '15m', 0) macro with $__unixEpochFilter, because the timeGroup doesn’t work with Epoch time format.

I would like to use the fillvalue parameter in the time grouping ($__timeGroup(dateColumn,‘5m’[, fillvalue])), because not all times have a value.

I tried this:

SELECT $__timeGroup(ts_quarter, '15m', 0) AS time_sec,

The error message:

Invalid type for column time/time_sec, must be of type timestamp or unix timestamp, got: <nil> <nil>

It’s working but don’t have fill null feature:

SELECT CAST(CAST(ts_quarter/900 AS signed)*900 AS signed) AS time_sec,

How do I convert the Epoc time format to the appropriate format for the timeGroup macro?

Link for doc: http://docs.grafana.org/features/datasources/mysql/#macros