diff --git a/Lib/unittest/loader.py b/Lib/unittest/loader.py
index f0cc157..360a41e 100644
--- a/Lib/unittest/loader.py
+++ b/Lib/unittest/loader.py
@@ -192,7 +192,7 @@
                 top_part = start_dir.split('.')[0]
                 start_dir = os.path.abspath(os.path.dirname((the_module.__file__)))
                 if set_implicit_top:
-                    self._top_level_dir = os.path.abspath(os.path.dirname(os.path.dirname(sys.modules[top_part].__file__)))
+                    self._top_level_dir = self._get_directory_containing_module(top_part)
                     sys.path.remove(top_level_dir)
 
         if is_not_importable:
@@ -201,6 +201,18 @@
         tests = list(self._find_tests(start_dir, pattern))
         return self.suiteClass(tests)
 
+    def _get_directory_containing_module(self, module_name):
+        module = sys.modules[module_name]
+        full_path = os.path.abspath(module.__file__)
+
+        if os.path.basename(full_path).lower().startswith('__init__.py'):
+            return os.path.dirname(os.path.dirname(full_path))
+        else:
+            # here we have been given a module rather than a package - so
+            # all we can do is search the *same* directory the module is in
+            # should an exception be raised instead
+            return os.path.dirname(full_path)
+
     def _get_name_from_path(self, path):
         path = os.path.splitext(os.path.normpath(path))[0]