help needed on SPI_modifytuple.

I'm trying to sipmply modify a tuple before it will be inserted in the db.
The problem is that when it try to insert the modified tuple with
SPI_modifytuple all i get is a SPI_ERROR_ARGUMENT negative (-6) .
Could someone help me? i'm reallygoing crazy.

What i did is just a fire-before C trigger that acts like this:

#include <postgres.h>
#include <executor/spi.h> /* this is what you need to work with SPI */
#include <commands/trigger.h> /* ... and triggers */
#include <string.h>
#include <utils/builtins.h>

extern Datum trgupdana(PG_FUNCTION_ARGS);
char *checkFieldData(char *);

PG_FUNCTION_INFO_V1(trgupdana);

Datum
trgupdana(PG_FUNCTION_ARGS)
{
TriggerData *trigdata = (TriggerData *) fcinfo->context;
TupleDesc tupdesc;
HeapTuple rettuple,
oldtuple,
newtuple;
char *rs1,*rs2,*rs3,*relname;
int ret, i,j;
bool isnull;
Relation rel;

/* make sure it's called as a trigger at all */
if (!CALLED_AS_TRIGGER(fcinfo)) elog(ERROR, "trgchkneg: not called
by trigger manager\n");

if (TRIGGER_FIRED_BEFORE(trigdata->tg_event) &&
TRIGGER_FIRED_FOR_ROW(trigdata->tg_event)) //trigger fires when called
for rows and before updating
{
tupdesc = trigdata->tg_relation->rd_att;
oldtuple = trigdata->tg_trigtuple;
newtuple = trigdata->tg_newtuple;
rettuple=NULL;
rel = trigdata->tg_relation;
relname= SPI_getrelname(rel);

if ((ret = SPI_connect()) < 0) // SPI
manager initialization
elog(NOTICE, "trgCheckNeg : SPI_connect returned: %d", ret);

rs1=SPI_getvalue(oldtuple,tupdesc,5);
rs2=SPI_getvalue(oldtuple,tupdesc,6);

elog(NOTICE,"%s,%s",rs1,rs2);

int attnum;
Datum new_value;

rs3=(char*) malloc(80);
sprintf(rs3,"");

if(rs1[strlen(rs1)-1]=='&'){
rs3=strncat(rs3,rs1,strlen(rs1)-1);
rs3=strcat(rs3,"E ");
}
else if(rs1[strlen(rs1)-1]==',')
{
rs3=strncat(rs3,rs1,strlen(rs1)-1);
rs3=strcat(rs3,", ");
} else {
rs3=rs1;
}

elog(NOTICE,"1:%s",rs3);

if(strlen(rs2)!=0){
rs3=strcat(rs3,rs2);
}

elog(NOTICE,"2:%s",rs3);

new_value=DirectFunctionCall1(textin,PointerGetDatum(checkFieldData(rs3)));

attnum=SPI_fnumber(tupdesc,"ARGSL1");

if(rel==NULL) elog(NOTICE,"rel NULL");
if(&attnum==NULL) elog(NOTICE,"attnum NULL");
if(&new_value==NULL) elog(NOTICE,"new_value NULL");
if(&isnull==NULL) elog(NOTICE,"isnull NULL");

rettuple=SPI_modifytuple(rel,newtuple,1,&attnum,&new_value,&isnull);
if(rettuple==NULL){
elog(ERROR,"trgupdana (%s):%d returned by
SPI_modifytuple",relname,SPI_result);
}
return PointerGetDatum(rettuple);
}
return PointerGetDatum(oldtuple);
}

char *checkFieldData(char *valueToCheck){
int i=0;
for(i=0;i<strlen(valueToCheck);i++)
{
if(valueToCheck[i]=='&') valueToCheck[i]='E';
else if(valueToCheck[i]=='\'') valueToCheck[i]=96;
}

return(valueToCheck);
}

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