diff --git a/Lib/ftplib.py b/Lib/ftplib.py
index ebc88df3b5a..2ccfbcbffc7 100644
--- a/Lib/ftplib.py
+++ b/Lib/ftplib.py
@@ -392,7 +392,7 @@ class FTP:
         The callback function (2nd argument) is called for each line,
         with trailing CRLF stripped.  This creates a new port for you.
         print_line() is the default callback.'''
-        if not callable(callback): callback = print_line
+        if callback is None: callback = print_line
         resp = self.sendcmd('TYPE A')
         conn = self.transfercmd(cmd)
         fp = conn.makefile('rb')