diff --git a/graphene/relay/connection.py b/graphene/relay/connection.py
index f6d3296d..8581a4b5 100644
--- a/graphene/relay/connection.py
+++ b/graphene/relay/connection.py
@@ -152,7 +152,7 @@ class IterableConnectionField(Field):
             return resolved
 
         assert isinstance(resolved, Iterable), (
-            "Resolved value from the connection field has to be iterable or instance of {}. "
+            "Resolved value from the connection field has to be an iterable or instance of {}. "
             'Received "{}"'
         ).format(connection_type, resolved)
         connection = connection_from_array(