diff --git a/sleekxmpp/features/feature_mechanisms/stanza/auth.py b/sleekxmpp/features/feature_mechanisms/stanza/auth.py
index d2a981f9..a366f72a 100644
--- a/sleekxmpp/features/feature_mechanisms/stanza/auth.py
+++ b/sleekxmpp/features/feature_mechanisms/stanza/auth.py
@@ -41,7 +41,10 @@ class Auth(StanzaBase):
 
     def set_value(self, values):
         if not self['mechanism'] in self.plain_mechs:
-            self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+            if values:
+                self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+            else:
+                self.xml.text = '='
         else:
             self.xml.text = bytes(values).decode('utf-8')
 
diff --git a/sleekxmpp/features/feature_mechanisms/stanza/challenge.py b/sleekxmpp/features/feature_mechanisms/stanza/challenge.py
index 82af869f..342463e1 100644
--- a/sleekxmpp/features/feature_mechanisms/stanza/challenge.py
+++ b/sleekxmpp/features/feature_mechanisms/stanza/challenge.py
@@ -33,7 +33,10 @@ class Challenge(StanzaBase):
         return base64.b64decode(bytes(self.xml.text))
 
     def set_value(self, values):
-        self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+        if values:
+            self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+        else:
+            self.xml.text = '='
 
     def del_value(self):
         self.xml.text = ''
diff --git a/sleekxmpp/features/feature_mechanisms/stanza/response.py b/sleekxmpp/features/feature_mechanisms/stanza/response.py
index 45bb8207..65f35888 100644
--- a/sleekxmpp/features/feature_mechanisms/stanza/response.py
+++ b/sleekxmpp/features/feature_mechanisms/stanza/response.py
@@ -33,7 +33,10 @@ class Response(StanzaBase):
         return base64.b64decode(bytes(self.xml.text))
 
     def set_value(self, values):
-        self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+        if values:
+            self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
+        else:
+            self.xml.text = '='
 
     def del_value(self):
         self.xml.text = ''