It doesn't. COUNT counts when the argument is true. CL:NIL is not true. The correct way to count the number of things, here symbols, is: (loop for s being e

It doesn't. COUNT counts when the argument is true. CL:NIL is not true. The correct way to count the number of things, here symbols, is: (loop for s being each symbol of (find-package :cl) count t) ; -> 978 or use: (loop for s being each symbol of (find-package :cl) sum 1) The Common Lisp standard says this in 6.1.3 Value Accumulation Clauses: > The count construct counts the number of times that the supplied form returns true. So the original code is NOT equivalent to (let ((count 0)) (do-symbols (symbol (find-package :cl) count) (incf count))) ; -> 978 but to (let ((count 0)) (do-symbols (symbol (find-package :cl) count) (when symbol ; test if true (incf count)))) ; -> 977