diff --git a/imagekit/admin.py b/imagekit/admin.py
index cc24d29..ebab76c 100644
--- a/imagekit/admin.py
+++ b/imagekit/admin.py
@@ -21,11 +21,11 @@ class AdminThumbnail(object):
         self.template = template
 
     def __call__(self, obj):
-        thumbnail = getattr(obj, self.image_field, None)
-
-        if not thumbnail:
-            raise Exception('The property {0} is not defined on {1}.'.format(
-                obj, self.image_field))
+        try:
+            thumbnail = getattr(obj, self.image_field)
+        except AttributeError:
+            raise Exception('The property %s is not defined on %s.' % \
+                    (self.image_field, obj.__class__.__name__))
 
         original_image = getattr(thumbnail, 'source_file', None) or thumbnail
         template = self.template or 'imagekit/admin/thumbnail.html'
diff --git a/imagekit/templates/imagekit/admin/thumbnail.html b/imagekit/templates/imagekit/admin/thumbnail.html
index 6531391..adaa89f 100644
--- a/imagekit/templates/imagekit/admin/thumbnail.html
+++ b/imagekit/templates/imagekit/admin/thumbnail.html
@@ -1,3 +1,5 @@
-<a href="{{ model.get_absolute_url|default:original_image.url }}">
-	<img src="{{ thumbnail.url }}">
-</a>
\ No newline at end of file
+{% if thumbnail %}
+	<a href="{{ model.get_absolute_url|default:original_image.url }}">
+		<img src="{{ thumbnail.url }}">
+	</a>
+{% endif %}
\ No newline at end of file
diff --git a/imagekit/utils.py b/imagekit/utils.py
index b6ca548..07cd532 100644
--- a/imagekit/utils.py
+++ b/imagekit/utils.py
@@ -97,9 +97,10 @@ def _extension_to_format(extension):
 
 
 def _format_to_extension(format):
-    for k, v in Image.EXTENSION.iteritems():
-        if v == format.upper():
-            return k
+    if format:
+        for k, v in Image.EXTENSION.iteritems():
+            if v == format.upper():
+                return k
     return None
 
 
@@ -121,11 +122,13 @@ def format_to_extension(format):
     """Returns the first extension that matches the provided format.
 
     """
-    extension = _format_to_extension(format)
-    if not extension and _preinit_pil():
-        extension = _format_to_extension(format)
-    if not extension and _init_pil():
+    extension = None
+    if format:
         extension = _format_to_extension(format)
+        if not extension and _preinit_pil():
+            extension = _format_to_extension(format)
+        if not extension and _init_pil():
+            extension = _format_to_extension(format)
     if not extension:
         raise UnknownFormatError(format)
     return extension