Memory Allocation Error

I wrote and compiled and linked a C function, add_float, which returns
float in C like following:

PG_FUNCTION_INFO_V1(add_float);

Datum
add_float(PG_FUNCTION_ARGS)
{
float arg = PG_GETARG_FLOAT8(0);

PG_RETURN_FLOAT8(arg + 10);
}

After having loaded it into database, executed the SQL command, "select
ad_float(2.90)", but one error occured. The error message is "Error:
invalid memory alloc request size 4294967290" .

However I changed the above function as following:

PG_FUNCTION_INFO_V1(add_integer);
Datum
add_integer(PG_FUNCTION_ARGS)
{
int32 arg = PG_GETARG_INT32(0);

PG_RETURN_INT32(arg + 10);
}

and executed the SQL command, "select add_integer(5)", and the result is:
add_integer
----------
15

In another word, the function can NOT return float BUT return integer.
Why? Please help me.

Thank you very much.