diff --git a/src/zeep/xsd/types/builtins.py b/src/zeep/xsd/types/builtins.py
index 5d93ef8ac..48d89889c 100644
--- a/src/zeep/xsd/types/builtins.py
+++ b/src/zeep/xsd/types/builtins.py
@@ -339,6 +339,8 @@ class Base64Binary(BuiltinType, AnySimpleType):
 
     @check_no_collection
     def xmlvalue(self, value):
+        if isinstance(value, six.string_types):
+            return value
         return base64.b64encode(value)
 
     def pythonvalue(self, value):
diff --git a/tests/test_xsd_builtins.py b/tests/test_xsd_builtins.py
index 97b22e3ef..373b70713 100644
--- a/tests/test_xsd_builtins.py
+++ b/tests/test_xsd_builtins.py
@@ -222,8 +222,8 @@ def test_pythonvalue(self):
 class TestgYear:
     def test_xmlvalue(self):
         instance = builtins.gYear()
-        instance.xmlvalue((2001, None)) == "2001"
-        instance.xmlvalue((2001, pytz.utc)) == "2001Z"
+        assert instance.xmlvalue((2001, None)) == "2001"
+        assert instance.xmlvalue((2001, pytz.utc)) == "2001Z"
 
     def test_pythonvalue(self):
         instance = builtins.gYear()
@@ -313,6 +313,10 @@ class TestBase64Binary:
     def test_xmlvalue(self):
         instance = builtins.Base64Binary()
         assert instance.xmlvalue(b"hoi") == b"aG9p"
+        assert (
+            instance.xmlvalue("aG9p")
+            == "aG9p"
+        )
 
     def test_pythonvalue(self):
         instance = builtins.Base64Binary()