Mail about typecast

Dear friends,
   I want to run a query :

vikrant=> select date_part('epoch','12 hours'::timespan)::float4;
It returns a error message
ERROR:  function dtof(float8) does not exist

But i want to use typecast since the result of this query will be used
to update a field of type float4.

I have checked the pg_proc system table it has a function dtof with
float8 as argument i have checked the list of functions also and
'dtof(float8)' is shown in that list also.
can anyone solve this problem. I can solve the problem by using a c
program but i want to know when the function dtof(float8) is there in
pg_proc table then why it is not doing a typecasting.
Thanks in advance for your kind suggestion.

Thanks & regards,
Vikrant.