diff --git a/news/7.bugfix b/news/7.bugfix
new file mode 100644
index 0000000..0479f09
--- /dev/null
+++ b/news/7.bugfix
@@ -0,0 +1,2 @@
+- Use a better type check in the payload parser.
+  [Rotonen]
diff --git a/plone/rfc822/_utils.py b/plone/rfc822/_utils.py
index 403c8b4..8bcd098 100644
--- a/plone/rfc822/_utils.py
+++ b/plone/rfc822/_utils.py
@@ -246,7 +246,7 @@ def initializeObject(context, fields, message, defaultCharset="utf-8"):
         return
 
     # A single payload is a string, multiparts are lists
-    if isinstance(payloads, str):
+    if isinstance(payloads, six.string_types):
         if len(primary) != 1:
             raise ValueError(
                 "Got a single string payload for message, but no primary "