Wrong result: "unsigned_col = '-1'" returns the row with value 0 when the column is indexed

Comparing an indexed DOUBLE UNSIGNED column with the string constant '-1' returns the row whose value is 0. 0 = '-1' is false. The numeric constant -1 and the same query with IGNORE INDEX both return the correct empty set.

How to reproduce

CREATE TABLE t(c DOUBLE UNSIGNED, KEY(c));

INSERT INTO t VALUES (0), (5);

SELECT 0 = '-1';

-- 0

SELECT c FROM t IGNORE INDEX (c) WHERE c = '-1';

-- Empty set   correct

SELECT c FROM t WHERE c = -1;

-- Empty set   correct

SELECT c FROM t WHERE c = '-1';

-- expected: Empty set

-- actual:   0

Notes

INT UNSIGNED PRIMARY KEY, REAL UNSIGNED PRIMARY KEY and DECIMAL UNSIGNED with a secondary key did not show the problem in my tests; DOUBLE UNSIGNED with a secondary key does.