Problem with DirectFunctionCall3(array_in,...)

Hi!

At the moment I'm trying to create an own datatype "timeseries", which
should contain among others an array of float8 values.

As far as I know, float8 is a base type of postgres and therefore I
shouldn't need to create an own array-type.

In my own C-function "timeseries_in()" I would like to use "array_in()"
implemented in arrayfuncs.c. To insert a timestamp with timezone, I'm
using

DirectFunctionCall3(timestamptz_in, CStringGetDatum(zeitstempel),
ObjectIdGetDatum(InvalidOid), Int32GetDatum(-1));

and it works fine... Trying the same with

DirectFunctionCall3(array_in, CStringGetDatum(arraystring),
ObjectIdGetDatum(20), Int32GetDatum(-1));

unfortunately smashes my postmaster...

Having done a lot of research, I can't spot the mistake. Does anyone
know the problem or have a workaround for me?

Please tell me, if you want me to supply my source code or anything
else...

MfG
Jessica