diff --git a/skimage/viewer/plugins/base.py b/skimage/viewer/plugins/base.py
index 07b74f22..e4bcd17b 100644
--- a/skimage/viewer/plugins/base.py
+++ b/skimage/viewer/plugins/base.py
@@ -80,7 +80,7 @@ class Plugin(QtGui.QDialog):
 
     def add_argument(self, name, low, high, callback, **kwargs):
         name, slider = self.add_slider(name, low, high, callback, **kwargs)
-        self.arguments[name] = slider
+        self.arguments.append(slider)
 
     def add_keyword_argument(self, name, low, high, callback, **kwargs):
         name, slider = self.add_slider(name, low, high, callback, **kwargs)
diff --git a/skimage/viewer/plugins/canny.py b/skimage/viewer/plugins/canny.py
index 25500e01..376638e5 100644
--- a/skimage/viewer/plugins/canny.py
+++ b/skimage/viewer/plugins/canny.py
@@ -1,4 +1,5 @@
 from skimage.filter import canny
+
 from .overlayplugin import OverlayPlugin