diff --git a/haul/extenders/pipeline/google.py b/haul/extenders/pipeline/google.py
index 226f701..306e9e0 100644
--- a/haul/extenders/pipeline/google.py
+++ b/haul/extenders/pipeline/google.py
@@ -5,7 +5,7 @@
 
 def blogspot_s1600_extender(pipeline_index,
                             finder_image_urls,
-                            extender_image_urls=[],
+                            extender_image_urls=None,
                             *args, **kwargs):
     """
     Example:
@@ -13,6 +13,8 @@ def blogspot_s1600_extender(pipeline_index,
     to
     http://1.bp.blogspot.com/-S97wTYQKbrY/UkWukhKhTKI/AAAAAAAAJ0g/fcRDiqVC8Us/s1600/aaPOP+001.jpg
     """
+    if extender_image_urls is None:
+        extender_image_urls = []
 
     now_extender_image_urls = []
 
@@ -32,7 +34,7 @@ def blogspot_s1600_extender(pipeline_index,
 
 def ggpht_s1600_extender(pipeline_index,
                          finder_image_urls,
-                         extender_image_urls=[],
+                         extender_image_urls=None,
                          *args, **kwargs):
     """
     Example:
@@ -40,6 +42,8 @@ def ggpht_s1600_extender(pipeline_index,
     to
     http://lh4.ggpht.com/-fFi-qJRuxeY/UjwHSOTHGOI/AAAAAAAArgE/SWTMT-hXzB4/s1600/Celeber-ru-Emma-Watson-Net-A-Porter-The-Edit-Magazine-Photoshoot-2013-01.jpg
     """
+    if extender_image_urls is None:
+        extender_image_urls = []
 
     now_extender_image_urls = []
 
@@ -59,7 +63,7 @@ def ggpht_s1600_extender(pipeline_index,
 
 def googleusercontent_s1600_extender(pipeline_index,
                                      finder_image_urls,
-                                     extender_image_urls=[],
+                                     extender_image_urls=None,
                                      *args, **kwargs):
     """
     Example:
@@ -67,6 +71,8 @@ def googleusercontent_s1600_extender(pipeline_index,
     to
     https://lh6.googleusercontent.com/-T6V-utZHzbE/Ukjn-1MDOSI/AAAAAAAAA3g/H6Qcw1zt4n0/s1600/2101_aa2cac09d1c6431b8a635d61cd9c4471.jpg
     """
+    if extender_image_urls is None:
+        extender_image_urls = []
 
     now_extender_image_urls = []
 
diff --git a/haul/extenders/pipeline/pinterest.py b/haul/extenders/pipeline/pinterest.py
index 24e30a8..c6eedf1 100644
--- a/haul/extenders/pipeline/pinterest.py
+++ b/haul/extenders/pipeline/pinterest.py
@@ -5,7 +5,7 @@
 
 def original_image_extender(pipeline_index,
                             finder_image_urls,
-                            extender_image_urls=[],
+                            extender_image_urls=None,
                             *args, **kwargs):
     """
     Example:
@@ -15,6 +15,8 @@ def original_image_extender(pipeline_index,
     to
     http://media-cache-ec0.pinimg.com/originals/50/9b/bd/509bbd5c6543d473bc2b49befe75f4c6.jpg
     """
+    if extender_image_urls is None:
+        extender_image_urls = []
 
     now_extender_image_urls = []
 
diff --git a/haul/extenders/pipeline/tumblr.py b/haul/extenders/pipeline/tumblr.py
index 745d770..af39a14 100644
--- a/haul/extenders/pipeline/tumblr.py
+++ b/haul/extenders/pipeline/tumblr.py
@@ -5,7 +5,7 @@
 
 def media_1280_extender(pipeline_index,
                         finder_image_urls,
-                        extender_image_urls=[],
+                        extender_image_urls=None,
                         *args, **kwargs):
     """
     Example:
@@ -14,6 +14,8 @@ def media_1280_extender(pipeline_index,
     to
     http://25.media.tumblr.com/3f5f10d7216f1dd5eacb5eb3e302286a/tumblr_mtpcwdzKBT1qh9n5lo1_1280.png
     """
+    if extender_image_urls is None:
+        extender_image_urls = []
 
     now_extender_image_urls = []
 
diff --git a/haul/extenders/pipeline/wordpress.py b/haul/extenders/pipeline/wordpress.py
index c227f3b..d6e6528 100644
--- a/haul/extenders/pipeline/wordpress.py
+++ b/haul/extenders/pipeline/wordpress.py
@@ -5,7 +5,7 @@
 
 def original_image_extender(pipeline_index,
                             finder_image_urls,
-                            extender_image_urls=[],
+                            extender_image_urls=None,
                             *args, **kwargs):
     """
     Example:
@@ -15,6 +15,8 @@ def original_image_extender(pipeline_index,
     http://fashion-fever.nl/wp-content/upload/2013/09/DSC_0058.jpg
     http://www.wendyslookbook.com/wp-content/uploads/2013/09/Morning-Coffee-Run-7.jpg
     """
+    if extender_image_urls is None:
+        extender_image_urls = []
 
     now_extender_image_urls = []
 
diff --git a/haul/finders/pipeline/css.py b/haul/finders/pipeline/css.py
index c56579d..00698fa 100644
--- a/haul/finders/pipeline/css.py
+++ b/haul/finders/pipeline/css.py
@@ -5,7 +5,7 @@
 
 def background_image_finder(pipeline_index,
                             soup,
-                            finder_image_urls=[],
+                            finder_image_urls=None,
                             *args, **kwargs):
     """
     Find image URL in background-image
@@ -15,6 +15,8 @@ def background_image_finder(pipeline_index,
     to
     http://distilleryimage10.ak.instagram.com/bde04558a43b11e28e5d22000a1f979a_7.jpg
     """
+    if finder_image_urls is None:
+        finder_image_urls = []
 
     now_finder_image_urls = []
 
diff --git a/haul/finders/pipeline/html.py b/haul/finders/pipeline/html.py
index 161aec5..2533382 100644
--- a/haul/finders/pipeline/html.py
+++ b/haul/finders/pipeline/html.py
@@ -5,11 +5,13 @@
 
 def img_src_finder(pipeline_index,
                    soup,
-                   finder_image_urls=[],
+                   finder_image_urls=None,
                    *args, **kwargs):
     """
     Find image URL in <img>'s src attribute
     """
+    if finder_image_urls is None:
+        finder_image_urls = []
 
     now_finder_image_urls = []
 
@@ -29,11 +31,13 @@ def img_src_finder(pipeline_index,
 
 def a_href_finder(pipeline_index,
                   soup,
-                  finder_image_urls=[],
+                  finder_image_urls=None,
                   *args, **kwargs):
     """
     Find image URL in <a>'s href attribute
     """
+    if finder_image_urls is None:
+        finder_image_urls = []
 
     now_finder_image_urls = []
 
diff --git a/tests/test.py b/tests/test.py
index e6298c6..c0fae39 100644
--- a/tests/test.py
+++ b/tests/test.py
@@ -174,11 +174,13 @@ def test_find_html_document(self):
 
         def img_data_src_finder(pipeline_index,
                                 soup,
-                                finder_image_urls=[],
+                                finder_image_urls=None,
                                 *args, **kwargs):
             """
             Find image URL in <img>'s data-src attribute
             """
+            if finder_image_urls is None:
+                finder_image_urls = []
 
             now_finder_image_urls = []