Error calling PG_RETURN_NULL()

Hi! I'm working on an implementation for a new data type (PostgreSQL
version 9.1 beta 3 on Windows 7 32 bits), according to the following rules:
- 1. NULL values are stored as is;
- 2. character strings (up to 16 bytes) are stored without leading or
trailing spaces;
- 3. empty character strings are stored as NULL values.

Using the extension support for new data types, the following source
code in C was written.

cs_type.h
--------------
#include "postgres.h"
#include "fmgr.h"

///
/// Export DLL functions.
///
#if defined(_WIN32)
#define DLLEXPORT __declspec(dllexport)
#else
#define DLLEXPORT
#endif

///
/// PostgreSQL magic block.
///
#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

///
/// Function prototypes.
///
DLLEXPORT Datum cs_type_in(PG_FUNCTION_ARGS);
DLLEXPORT Datum cs_type_out(PG_FUNCTION_ARGS);

cs_type.c
--------------
#include "cs_type.h"

///
/// Version-1 calling convention.
/// Input function.
///
PG_FUNCTION_INFO_V1(cs_type_in);
DLLEXPORT Datum cs_type_in(PG_FUNCTION_ARGS) {
char *cp1; // Trimming routine - for parsing the whole string.
char *cp2; // Trimming routine - for shifting & padding.
VarChar *v; // Return value.

char *c1 = PG_GETARG_CSTRING(0);

char *c2 = (char *)palloc(strlen(c1) + 1);
strcpy(c2, c1);

///
/// Trimming routine.
(http://stackoverflow.com/questions/656542/trim-a-string-in-c)
///
// skip leading spaces, shift remaining chars
for(cp1 = c2;isspace(*cp1);cp1++ ) // skip leading spaces, via cp1
;
for(cp2 = c2;*cp1;cp1++,cp2++) // shift left remaining chars, via cp2
*cp2 = *cp1;
*cp2-- = 0; // mark new end of string for str
// replace trailing spaces with '\0'
while(cp2 > c2 && isspace(*cp2))
*cp2-- = 0; // pad with '\0's

if(strlen(c2) == 0) { // Empty string: return NULL.
PG_RETURN_NULL();
}
else if(strlen(c2) > 16) { // Value too long: error.
ereport(ERROR, (errcode(ERRCODE_STRING_DATA_LENGTH_MISMATCH),
errmsg("value too long for type cs_type"), errhint("type cs_type
supports up to 16 bytes")));
}

///
/// Result as varchar.
///
v = (VarChar *)palloc(VARHDRSZ + strlen(c2) + 1);
SET_VARSIZE(v, VARHDRSZ + strlen(c2) + 1);
strcpy(VARDATA(v), c2);
PG_RETURN_VARCHAR_P(v);
}

///
/// Version-1 calling convention.
/// Output function.
///
PG_FUNCTION_INFO_V1(cs_type_out);
DLLEXPORT Datum cs_type_out(PG_FUNCTION_ARGS) {

VarChar *v = PG_GETARG_VARCHAR_P(0);

///
/// Result as cstring.
///
char *c = (char *)palloc(VARSIZE(v) - VARHDRSZ + 1);
strcpy(c, VARDATA(v));
PG_RETURN_CSTRING(c);
}

On the PostgreSQL side, the following objects were created.

CREATE OR REPLACE FUNCTION cs_type_in(cstring)
RETURNS cs_type AS
'$libdir/cs_type', 'cs_type_in'
LANGUAGE c STRICT
COST 1;
ALTER FUNCTION cs_type_in(cstring) OWNER TO postgres;

CREATE OR REPLACE FUNCTION cs_type_out(cs_type)
RETURNS cstring AS
'$libdir/cs_type', 'cs_type_out'
LANGUAGE c STRICT
COST 1;
ALTER FUNCTION cs_type_out(cs_type) OWNER TO postgres;

CREATE TYPE cs_type (
INPUT = cs_type_in(cstring),
OUTPUT = cs_type_out(cs_type),
LIKE = varchar
);
ALTER TYPE cs_type OWNER TO postgres;

CREATE TABLE test_cs_type
(
cs_value cs_type
)
WITH (
OIDS=FALSE
);
ALTER TABLE test_cs_type OWNER TO postgres;

When called directly, the function cs_type_in(cstring) works as
expected, attending the three rules described above. For example:

SELECT cs_type_in('TEST'); -- returns 'TEST'
SELECT cs_type_in(NULL); -- returns NULL
SELECT cs_type_in(''); -- returns NULL
SELECT cs_type_in(' '); -- returns NULL

However, on INSERT clauses, only the rules 1 and 2 work; an attempt to
insert an empty string (or a string with white spaces) generates an
error. For example:

INSERT INTO test_cs_type VALUES (NULL); -- works fine
INSERT INTO test_cs_type VALUES ('TEST'); -- works fine
INSERT INTO test_cs_type VALUES (''); -- error!
INSERT INTO test_cs_type VALUES (' '); -- error!

The error message displayed is:

ERRO: input function 49344 returned NULL
LINE 1: INSERT INTO dicom_data.test_cs_type VALUES (' ');
^

********** Error **********

ERRO: input function 49344 returned NULL
SQL state: XX000
Character: 45

It seems like the call to PG_RETURN_NULL() on the input function is
causing the error. Is this the correct behaviour? There's another way to
return a NULL value as the result of a data type's input function?

Best regards,
Alexandre