computation of dmath.log(13)

I found a strange behavior of dmath.log.
If I set the precision to 12 (decimal.getcontext().prec=12),
dmath.log(13) returns Decimal("2.56494935746").
But, if I set the precision to 20, dmath.log(13) never halts.

Is it a bug of dmath.log, or of decimal module of Python?