diff --git a/doc/build.py b/doc/build.py
index 214f01ab..c94344b9 100755
--- a/doc/build.py
+++ b/doc/build.py
@@ -39,6 +39,7 @@ def build_docs():
   pip_install('michaeljones/breathe',
               '511b0887293e7c6b12310bb61b3659068f48f0f4')
   print(check_output(['sphinx-build', '--version']))
+  print('PATH:', os.environ['PATH'])
   # Build docs.
   cmd = ['doxygen', '-']
   p = Popen(cmd, stdin=PIPE)