diff --git a/lib/matplotlib/backends/backend_webagg.py b/lib/matplotlib/backends/backend_webagg.py
index 9583906ab2fe..fe0310787376 100644
--- a/lib/matplotlib/backends/backend_webagg.py
+++ b/lib/matplotlib/backends/backend_webagg.py
@@ -17,7 +17,6 @@
 
 import datetime
 import errno
-import io
 import json
 import os
 import random
@@ -197,7 +196,7 @@ def get(self, fignum, fmt):
 
             self.set_header('Content-Type', mimetypes.get(fmt, 'binary'))
 
-            buff = io.BytesIO()
+            buff = six.BytesIO()
             manager.canvas.print_figure(buff, format=fmt)
             self.write(buff.getvalue())
 
diff --git a/lib/matplotlib/backends/backend_webagg_core.py b/lib/matplotlib/backends/backend_webagg_core.py
index 8f971600d040..7a4bec1a325a 100644
--- a/lib/matplotlib/backends/backend_webagg_core.py
+++ b/lib/matplotlib/backends/backend_webagg_core.py
@@ -403,7 +403,8 @@ def get_javascript(cls, stream=None):
         for filetype, ext in sorted(FigureCanvasWebAggCore.
                                     get_supported_filetypes_grouped().
                                     items()):
-            extensions.append(ext[0])
+            if not ext[0] == 'pgf':  # pgf does not support BytesIO
+                extensions.append(ext[0])
         output.write("mpl.extensions = {0};\n\n".format(
             json.dumps(extensions)))