diff --git a/jedi/utils.py b/jedi/utils.py
index 2502620c..44f7c0d2 100644
--- a/jedi/utils.py
+++ b/jedi/utils.py
@@ -5,6 +5,8 @@ Utilities for end-users.
 from __future__ import absolute_import
 import __main__
 from collections import namedtuple
+import logging
+import traceback
 import re
 import os
 import sys
@@ -14,6 +16,9 @@ from jedi.api.helpers import get_on_completion_name
 from jedi import common
 
 
+READLINE_DEBUG = False
+
+
 def setup_readline(namespace_module=__main__):
     """
     Install Jedi completer to :mod:`readline`.
@@ -55,6 +60,13 @@ def setup_readline(namespace_module=__main__):
     bash).
 
     """
+    if READLINE_DEBUG:
+        logging.basicConfig(
+            filename='/tmp/jedi.log',
+            filemode='a',
+            level=logging.DEBUG
+        )
+
     class JediRL(object):
         def complete(self, text, state):
             """
@@ -70,6 +82,7 @@ def setup_readline(namespace_module=__main__):
                 sys.path.insert(0, os.getcwd())
                 # Calling python doesn't have a path, so add to sys.path.
                 try:
+                    logging.debug("Start REPL completion: " + repr(text))
                     interpreter = Interpreter(text, [namespace_module.__dict__])
 
                     lines = common.splitlines(text)
@@ -77,6 +90,9 @@ def setup_readline(namespace_module=__main__):
                     name = get_on_completion_name(interpreter._get_module(), lines, position)
                     before = text[:len(text) - len(name)]
                     completions = interpreter.completions()
+                except:
+                    logging.error("REPL Completion error:\n" + traceback.format_exc())
+                    raise
                 finally:
                     sys.path.pop(0)
 
diff --git a/test/test_api/test_interpreter.py b/test/test_api/test_interpreter.py
index be3f12d5..73773db1 100644
--- a/test/test_api/test_interpreter.py
+++ b/test/test_api/test_interpreter.py
@@ -156,4 +156,5 @@ class TestInterpreterAPI(TestCase):
         lambd = lambda x: 3
 
         self.check_interpreter_complete('foo(bar', locals(), ['bar'])
-        self.check_interpreter_complete('lambd(x', locals(), ['x'])
+        # TODO we're not yet using the Python3.5 inspect.signature, yet.
+        assert not jedi.Interpreter('lambd(x', [locals()]).completions()