Basic sql

Well I've had my coffee and I still can't get this to work:

SELECT COUNT(*) FROM a

      WHERE EXISTS (SELECT *=20
	  	      FROM b
			WHERE a.unit_id =3D b.unit_id
			and a.op_year =3D b.op_year
			and extract(month from a.op_date) BETWEEN  CASE
			    WHEN b.qtr =3D 1 THEN 1 AND 3
			    WHEN b.qtr =3D 2 THEN 4 AND 6
			    WHEN b.qtr =3D 3 THEN7 AND 9
			    WHEN b.qtr =3D 4 THEN 10 AND 12
			  END)

I've tried several variations of this (all that I can think of)i.e. quotes and parenthesis in all kinds of places, case to build entire last filter instead of just the '1 and 3' pieces. and it always returns:

ERROR at line 7:
ORA-00905: missing keyword

9.2 on windows

What simple thing am I missing now?

Thank you for the extra eyes!

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