why the bit limit of 2^24 in RealField?

[zimmerma]

sage: R = RealField(16777217)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

/users/caramel/zimmerma/.sage/temp/patate.loria.fr/31828/_users_caramel_zimmerm\
a__sage_init_sage_0.py in <module>()

/usr/local/sage-core2/local/lib/python2.6/site-packages/sage/rings/real_mpfr.so\
 in sage.rings.real_mpfr.RealField_constructor (sage/rings/real_mpfr.c:3723)()

/usr/local/sage-core2/local/lib/python2.6/site-packages/sage/rings/real_mpfr.so\
 in sage.rings.real_mpfr.RealField.__init__ (sage/rings/real_mpfr.c:3945)()

ValueError: prec (=16777217) must be >= 2 and <= 16777216.

Note that 2^{24 bits is only slightly above 5M digits, which is quite small (Fabrice Bellard recently computed 2700 billions of digits of Pi on a personal desktop, i.e., about 500,000 times more). of Pi}