RETURN QUERY generates error

Here is my function:

CREATE OR REPLACE FUNCTION "hg18"."get_genomeseq" (_chr varchar,
_byblocks boolean, _starts integer [], _ends integer []) RETURNS SETOF
text AS
$body$
DECLARE
_start integer;
_end integer;
_sequence text[];
_seq50 RECORD;
_seq text;
_q text;
BEGIN
FOR i IN 1..array_upper(_starts, 1) LOOP
_start := _starts[i];
_end := _ends[i];
_q :=
'SELECT start, sequence ' ||
'FROM hg18.genome ' ||
$$WHERE chr = 'chr' || '$$ || _chr::varchar || $$' $$ ||
'AND start >= floor(' || _start || '/50)*50 ' ||
'AND start < ' || _end;
--RAISE NOTICE 'Query is %', _q;
FOR _seq50 IN EXECUTE _q LOOP
IF _seq50.start < _start THEN
_sequence[i] := substring(_seq50.sequence, _start%_seq50.start);
ELSEIF _seq50.start >= _start AND _seq50.start + 49 <= _end THEN
_sequence[i] := _sequence[i] || _seq50.sequence;
ELSE
_sequence[i] := _sequence[i]
|| substring(_seq50.sequence, 1, _end%_seq50.start);
END IF;
END LOOP;
END LOOP;

IF _byblocks IS TRUE THEN
RETURN QUERY SELECT
regexp_split_to_table(array_to_string(_sequence, ','), E',');
ELSE
RETURN QUERY SELECT array_to_string(_sequence, '');
END IF;
RETURN;
END;
$body$
LANGUAGE 'plpgsql' IMMUTABLE CALLED ON NULL INPUT SECURITY INVOKER;

all is fine until the last IF. Both RETURN QUERY blocks generate
error: 'syntax error at or near SELECT ...' I feel that something
wrong with casting _sequence var but I can't figure out the exact
point.