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
leftremaining chars, via cp2        *cp2 = *cp1;    *cp2-- = 0; // mark new end of string for str    // replace
trailingspaces 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) { //
Valuetoo 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