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.