Error when defining a set returning function

Dear all

Since I was receiving an error when defining a set returning function, I
borrowed a function from PostgreSQL as follows

/* C definition */
typedef struct testState
{
int current;
int finish;
int step;
} testState;

/**
* test_srf(startval int, endval int, step int)
*/
PG_FUNCTION_INFO_V1(test_srf);
Datum test_srf(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
testState *fctx;
int result; /* the actual return value */

if (SRF_IS_FIRSTCALL())
{
/* Get input values */
int start = PG_GETARG_INT32(0);
int finish = PG_GETARG_INT32(1);
int step = PG_GETARG_INT32(2);
MemoryContext oldcontext;

/* create a function context for cross-call persistence */
funcctx = SRF_FIRSTCALL_INIT();

/* switch to memory context appropriate for multiple function calls */
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

/* quick opt-out if we get nonsensical inputs */
if (step <= 0 || start == finish)
{
funcctx = SRF_PERCALL_SETUP();
SRF_RETURN_DONE(funcctx);
}

/* allocate memory for function context */
fctx = (testState *) palloc0(sizeof(testState));
fctx->current = start;
fctx->finish = finish;
fctx->step = step;

funcctx->user_fctx = fctx;
MemoryContextSwitchTo(oldcontext);
}

/* stuff done on every call of the function */
funcctx = SRF_PERCALL_SETUP();

/* get state */
fctx = funcctx->user_fctx;

result = fctx->current;
fctx->current += fctx->step;
/* Stop when we have generated all values */
if (fctx->current > fctx->finish)
{
SRF_RETURN_DONE(funcctx);
}

SRF_RETURN_NEXT(funcctx, Int32GetDatum(result));
}

/* SQL definition */
CREATE OR REPLACE FUNCTION testSRF(startval int, endval int, step int)
RETURNS SETOF integer
AS 'MODULE_PATHNAME', 'test_srf'
LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;

When I execute this function I obtain

select testSRF(1,10, 2);
ERROR: unrecognized table-function returnMode: 257

select version();
PostgreSQL 13.2 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
9.3.0-17ubuntu1~20.04) 9.3.0, 64-bit

Any idea what could be wrong ?

Thanks for your help

Esteban