diff --git a/doc/build.py b/doc/build.py
index 412d407b..33a79baa 100755
--- a/doc/build.py
+++ b/doc/build.py
@@ -40,6 +40,7 @@ def build_docs():
               '511b0887293e7c6b12310bb61b3659068f48f0f4')
   print(check_output(['sphinx-build', '--version']))
   print('PATH:', os.environ['PATH'])
+  print(check_output(['which', 'sphinx-build']))
   # Build docs.
   cmd = ['doxygen', '-']
   p = Popen(cmd, stdin=PIPE)