I'm running the following SOQL statement and it always produces an unexpected error:

select week_in_year(ActivityDate), count(Id)
from Task
group by week_in_year(ActivityDate)

The error from a previous similar call was: "UNKNOWN_EXCEPTION: An unexpected error occurred. Please include this ErrorId if you contact support: 228937263-10791 (1171053875)"  However, the following code works fine:

select ActivityDate, count(Id)
from Task
group by ActivityDate

Is there something I'm missing or is this a quirk with week_in_year?  I'm using the 18.0 version of the SOAP partner API (https://www.salesforce.com/services/Soap/u/18.0).

Thanks!