diff --git a/cortex/webgl/serve.py b/cortex/webgl/serve.py
index 3be573ced..566906c04 100644
--- a/cortex/webgl/serve.py
+++ b/cortex/webgl/serve.py
@@ -31,7 +31,7 @@
 def make_base64(imgfile):
     with open(imgfile, 'rb') as img:
         mtype = mimetypes.guess_type(imgfile)[0]
-        imbytes = base64.encodestring(img.read())
+        imbytes = base64.encodebytes(img.read())
         data = imbytes.decode('utf-8').strip()
         return u"data:{mtype};base64,{data}".format(mtype=mtype, data=data)