ECL does not support docstrings for functions created with flet, labels or lambda. Why?

Hi!

I found that:

CL-USER> (let ((foo (lambda () "With docstring" 100500)))

       (princ (funcall foo))
       (terpri)
       (princ (documentation foo t)))

100500 NIL NIL

in ECL, but:

CL-USER> (let ((foo (lambda () "With docstring" 100500)))

       (princ (funcall foo))
       (terpri)
       (princ (documentation foo t)))

100500 With docstring "With docstring"

Why?

BTW, similar behaviour have ABCL, Allegro and CCL. But SBCL and CLISP work as expected.