diff --git a/nbconvert/filters/markdown_mistune.py b/nbconvert/filters/markdown_mistune.py
index ff209f372..a46be513f 100644
--- a/nbconvert/filters/markdown_mistune.py
+++ b/nbconvert/filters/markdown_mistune.py
@@ -160,7 +160,10 @@ def image(self, src, title, text):
         attachment_prefix = 'attachment:'
         if src.startswith(attachment_prefix):
             name = src[len(attachment_prefix):]
-            assert name in attachments, "missing attachment: {}".format(name)
+            
+            if not name in attachments:
+                raise InvalidNotebook("missing attachment: {}".format(name))
+            
             attachment = attachments[name]
             # we choose vector over raster, and lossless over lossy
             preferred_mime_types = ['image/svg+xml', 'image/png', 'image/jpeg']
diff --git a/nbconvert/nbconvertapp.py b/nbconvert/nbconvertapp.py
index e137a173d..97ca798b2 100755
--- a/nbconvert/nbconvertapp.py
+++ b/nbconvert/nbconvertapp.py
@@ -595,6 +595,9 @@ def initialize(self, argv=None):
     def default_export_format(self):
         return 'html'
 
+class InvalidNotebook(Exception):
+    pass
+
 #-----------------------------------------------------------------------------
 # Main entry point
 #-----------------------------------------------------------------------------