replacing a function called "isnull" reports an error

Hi, I created a function called "isnull" for test
purposes and when trying to replace it returns the
message belows :

-- Function: public.isnull(anyelement)

-- DROP FUNCTION public.isnull(anyelement);

CREATE OR REPLACE FUNCTION public.isnull(anyelement)
RETURNS bool AS
'
SELECT $1 is null;
'
LANGUAGE 'sql' VOLATILE;

ERROR: syntax error at or near "isnull" at character
120

Thanks.

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