Can't put LIKE parameter in parameter dict?

Can someone explain why these two seemingly identical queries return
different results?  Can't LIKE wildcards be given in parameter dictionaries?

    >>> c.execute('select distinct symbol, open_time, close_time,'
    ...           '                session_start, session_end, instrument'
    ...           '  from underlying_sessions'
    ...           '  where instrument = @i'
    ...           '    and symbol like "M%"',
    ...           {"@i": "E"})
    >>> len(c.fetchall())
    402
    >>> c.execute('select distinct symbol, open_time, close_time,'
    ...           '                session_start, session_end, instrument'
    ...           '  from underlying_sessions'
    ...           '  where instrument = @i'
    ...           '    and symbol like @s',
    ...           {"@i": "E",
    ...            "@s": "M%"})
    >>> len(c.fetchall())
    0

Thanks,

-- 
Skip Montanaro
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