diff --git a/nbrmd/contentsmanager.py b/nbrmd/contentsmanager.py
index df410e15a..93d5b0af7 100644
--- a/nbrmd/contentsmanager.py
+++ b/nbrmd/contentsmanager.py
@@ -27,7 +27,7 @@ def _reads(s, as_version, **kwargs):
 
 
 def check_formats(formats):
-    if isinstance(formats, str):
+    if not isinstance(formats, list):
         formats = formats.split(',')
 
     formats = [fmt if fmt.startswith('.') else '.' + fmt