PL/Python NameError: global name '<variable>' is not defined

I'm trying to create a PL/Python function for PostgreSQL.  Here's a simplified version that gets the error.

create or replace function eztest( ASIN varchar(10)) returns  integer AS
$$
plan = plpy.prepare("Select bookid from book where ASIN = $1", ["text"])
rv = plpy.execute(plan, [ ASIN ])
return rv["bookid"]
$$ language plpython2u;

Select eztest('B00DMYO2D2');

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When I call the function with the SELECT statement, it complains

ERROR:  NameError: global name 'ASIN' is not defined
CONTEXT:  Traceback (most recent call last):
  PL/Python function "eztest", line 3, in <module>
    rv = plpy.execute(plan, [ ASIN ])
PL/Python function "eztest"

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If I hardcode a value in, rather than using my variable, it works.  What am I doing wrong?

Thanks!