handle graphviz parsing errors more cleanly

incorporealcms/mdx/pydot.py

@@ -1,10 +1,13 @@
 """Serve dot diagrams inline."""
 import base64
+import logging
 import re
 
 import markdown
 import pydot
 
+logger = logging.getLogger(__name__)
+
 
 class InlinePydot(markdown.Extension):
     """Wrap the markdown prepcoressor."""
@@ -29,6 +32,9 @@ class InlinePydotPreprocessor(markdown.preprocessors.Preprocessor):
 
             # use pydot to turn the text into pydot
             graphs = pydot.graph_from_dot_data(dot_string)
+            if not graphs:
+                logger.debug("some kind of issue with parsed 'dot' %s", dot_string)
+                raise ValueError("error parsing dot text!")
 
             # encode the image and provide as an inline image in markdown
             encoded_image = base64.b64encode(graphs[0].create_png()).decode('ascii')