create gdb pretty printer for boost::multiprecision::mpfr_float_50?

I am trying to create a pretty-printer for boost::multiprecision::mpfr_float_50.

This is the code I have:

import gdb
import gdb.printing

class BoostMultiprecision(object):
    "Pretty Printer for boost::multiprecision::mpfr_float_50"

    def __init__(self, val):
        self.val = val
        self.double = gdb.lookup_type('double')

    def to_string(self):
        eval_string = "(*("+str(self.val.type)+"*)("+str(self.val.address)+")).convert_to<double>()"
        result = gdb.parse_and_eval(eval_string)
        return result.cast(self.double)

    def display_hint(self):
        return 'mpfr_float_50'

def build_pretty_printer_mpfr_float_50():
    pp = gdb.printing.RegexpCollectionPrettyPrinter("boost::multiprecision::mpfr_float_50")
    pp.add_printer('mpfr_float_50', '^boost::multiprecision::mpfr_float_50$', BoostMultiprecision)
    return pp

def register_printers():
    gdb.printing.register_pretty_printer(gdb.current_objfile(), build_pretty_printer_mpfr_float_50())

This is my .gdbinit:

python
import sys
sys.path.insert(1, '/path/to/script')
import gdb_pretty_printers
gdb_pretty_printers.register_printers()

When I try to print my variable, I just get the following:

m_backend = {<boost::multiprecision::backends::detail::mpfr_float_imp<50,
(boost::multiprecision::mpfr_allocation_type)1>> = {m_data = {{_mpfr_prec = 168, _mpfr_sign = 1, 
_mpfr_exp = -7, _mpfr_d = 0x5555558752d8}}}

How can I pretty print my boost::multiprecision::mpfr_float_50 as a double?