Missing data from secondary index when secondary index is against a compound PK member

Under 2.0.8, assuming the following schema:

CREATE TABLE foo (
a text,
b text,
c text,
d text,
PRIMARY KEY (a, b)
);

CREATE INDEX foo_b_idx ON foo (b);

If you do a delete against c or d but specify a and b in the where clause, it will remove the secondary index reference to b (verified that it removes it from the underlying SSTable as well).

Example:

cqlsh:test> insert into foo (a, b, c, d) values ('test','test2','to_be_deleted', 'test3');
cqlsh:test> select * from foo;

a | b | c | d
-----------------------+------
test | test2 | to_be_deleted | test3

(1 rows)

cqlsh:test> delete c from foo where a = 'test' and b = 'test2';

cqlsh:test> select * from foo where b = 'test2';

(0 rows)

cqlsh:test> select * from foo where a = 'test' and b = 'test2';

a | b | c | d
--------------+------
test | test2 | null | test3

(1 rows)

cqlsh:test> select * from foo;

a | b | c | d
--------------+------
test | test2 | null | test3

(1 rows)

Dropping and recreating the index in CQL brings the secondary index reference back.