diff --git a/jedi/evaluate/cache.py b/jedi/evaluate/cache.py
index bf7a5342..1d1d06f3 100644
--- a/jedi/evaluate/cache.py
+++ b/jedi/evaluate/cache.py
@@ -4,6 +4,8 @@
 - ``CachedMetaClass`` uses ``memoize_default`` to do the same with classes.
 """
 
+NO_DEFAULT = object()
+
 
 def memoize_default(default=None, evaluator_is_first_arg=False, second_arg_is_evaluator=False):
     """ This is a typical memoization decorator, BUT there is one difference:
@@ -32,7 +34,8 @@ def memoize_default(default=None, evaluator_is_first_arg=False, second_arg_is_ev
             if key in memo:
                 return memo[key]
             else:
-                memo[key] = default
+                if default is not NO_DEFAULT:
+                    memo[key] = default
                 rv = function(obj, *args, **kwargs)
                 memo[key] = rv
                 return rv
diff --git a/jedi/evaluate/imports.py b/jedi/evaluate/imports.py
index 6147d38d..b923b880 100644
--- a/jedi/evaluate/imports.py
+++ b/jedi/evaluate/imports.py
@@ -27,7 +27,7 @@ from jedi.evaluate import helpers
 from jedi import settings
 from jedi.common import source_to_unicode
 from jedi.evaluate import compiled
-from jedi.evaluate.cache import memoize_default
+from jedi.evaluate.cache import memoize_default, NO_DEFAULT
 
 
 class ModuleNotFound(Exception):
@@ -290,7 +290,7 @@ class _Importer(object):
             return evaluator.follow_path(iter(rest), [scope], scope)
         return [scope]
 
-    @memoize_default()
+    @memoize_default(NO_DEFAULT)
     def follow_file_system(self):
         if self.file_path:
             sys_path_mod = list(self.sys_path_with_modifications())
diff --git a/test/completion/imports.py b/test/completion/imports.py
index fc054b01..cd5193a9 100644
--- a/test/completion/imports.py
+++ b/test/completion/imports.py
@@ -303,3 +303,17 @@ recurse_class1.C.a
 # github #239 RecursionError
 #? ['a']
 recurse_class1.C().a
+
+# -----------------
+# Jedi debugging
+# -----------------
+
+# memoizing issues (check git history for the fix)
+import not_existing_import
+
+if not_existing_import:
+    a = not_existing_import
+else:
+    a = not_existing_import
+#? 
+a