diff --git a/python/spot/aux.py b/python/spot/aux.py
index d5bc34aaa..a85c26f30 100644
--- a/python/spot/aux.py
+++ b/python/spot/aux.py
@@ -23,6 +23,7 @@ Auxiliary functions for Spot's Python bindings
 
 from functools import lru_cache
 import subprocess
+import sys
 
 
 def extend(*classes):
@@ -50,9 +51,16 @@ def str_to_svg(str):
     """
     dot = subprocess.Popen(['dot', '-Tsvg'],
                            stdin=subprocess.PIPE,
-                           stdout=subprocess.PIPE)
-    res = dot.communicate(str)
-    return res[0].decode('utf-8')
+                           stdout=subprocess.PIPE,
+                           stderr=subprocess.PIPE)
+    stdout, stderr = dot.communicate(str)
+    if stderr:
+        print("Calling 'dot' for the conversion to SVG produced the message:\n"
+              + stderr.decode('utf-8'), file=sys.stderr)
+    ret = dot.wait()
+    if ret:
+        raise subprocess.CalledProcessError(ret, 'dot')
+    return stdout.decode('utf-8')
 
 
 def ostream_to_svg(ostr):
diff --git a/tests/Makefile.am b/tests/Makefile.am
index d4b68775c..93bd6a86a 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -284,6 +284,7 @@ TESTS_twa = \
 
 if USE_PYTHON
 TESTS_python = \
+  python/_aux.ipynb \
   python/acc_cond.ipynb \
   python/accparse.ipynb \
   python/accparse2.py \
diff --git a/tests/python/_aux.ipynb b/tests/python/_aux.ipynb
new file mode 100644
index 000000000..ed3527d71
--- /dev/null
+++ b/tests/python/_aux.ipynb
@@ -0,0 +1,73 @@
+{
+ "metadata": {
+  "name": "",
+  "signature": "sha256:6be10f711b59f226415bc570a040611b0a8c554e61bf92877b8af4040963e0ac"
+ },
+ "nbformat": 3,
+ "nbformat_minor": 0,
+ "worksheets": [
+  {
+   "cells": [
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "import spot.aux"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [],
+     "prompt_number": 1
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "Make sure `str_to_svg` reports errors from dot."
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "spot.aux.str_to_svg(b'syntax error')"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stderr",
+       "text": [
+        "Calling 'dot' for the conversion to SVG produced the message:\n",
+        "Error: <stdin>: syntax error in line 1 near 'syntax'\n",
+        "\n"
+       ]
+      },
+      {
+       "ename": "CalledProcessError",
+       "evalue": "Command 'dot' returned non-zero exit status 1",
+       "output_type": "pyerr",
+       "traceback": [
+        "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mCalledProcessError\u001b[0m                        Traceback (most recent call last)",
+        "\u001b[0;32m<ipython-input-3-294a2c217b9a>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mspot\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0maux\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mstr_to_svg\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34mb'syntax error'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
+        "\u001b[0;32m/home/adl/git/spot/python/spot/aux.py\u001b[0m in \u001b[0;36mstr_to_svg\u001b[0;34m(str)\u001b[0m\n\u001b[1;32m     60\u001b[0m     \u001b[0mret\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mdot\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mwait\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m     61\u001b[0m     \u001b[0;32mif\u001b[0m \u001b[0mret\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 62\u001b[0;31m         \u001b[0;32mraise\u001b[0m \u001b[0msubprocess\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mCalledProcessError\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mret\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m'dot'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m     63\u001b[0m     \u001b[0;32mreturn\u001b[0m \u001b[0mstdout\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mdecode\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m'utf-8'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m     64\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n",
+        "\u001b[0;31mCalledProcessError\u001b[0m: Command 'dot' returned non-zero exit status 1"
+       ]
+      }
+     ],
+     "prompt_number": 3
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [],
+     "language": "python",
+     "metadata": {},
+     "outputs": []
+    }
+   ],
+   "metadata": {}
+  }
+ ]
+}
\ No newline at end of file
diff --git a/tests/sanity/ipynb.pl b/tests/sanity/ipynb.pl
index a0709c4cc..dfebecf51 100755
--- a/tests/sanity/ipynb.pl
+++ b/tests/sanity/ipynb.pl
@@ -65,9 +65,8 @@ close(FD);
 open(FD, "$dir/../Makefile.am") or die "$!";
 while (<FD>)
 {
-    if (m:python/([\w-]+\.ipynb):)
+    if (m:python/([^_][\w-]*\.ipynb):)
     {
-	# print "$1 exist";
 	unless (exists $seen{$1})
 	{
 	    print STDERR "notebook $1 is not mentioned in tut.org";