diff --git a/taskflow/tests/utils.py b/taskflow/tests/utils.py
index 6c4d6ae3..8fcf9356 100644
--- a/taskflow/tests/utils.py
+++ b/taskflow/tests/utils.py
@@ -15,11 +15,10 @@
 #    under the License.
 
 import contextlib
-
-import collections
-import six
 import threading
 
+import six
+
 from taskflow import exceptions
 from taskflow.persistence.backends import impl_memory
 from taskflow import retry
@@ -317,11 +316,11 @@ class WaitForOneFromTask(SaveOrderTask):
 
     def __init__(self, name, wait_for, wait_states, **kwargs):
         super(WaitForOneFromTask, self).__init__(name, **kwargs)
-        if not isinstance(wait_for, collections.Iterable):
+        if isinstance(wait_for, six.string_types):
             self.wait_for = [wait_for]
         else:
             self.wait_for = wait_for
-        if not isinstance(wait_states, collections.Iterable):
+        if isinstance(wait_states, six.string_types):
             self.wait_states = [wait_states]
         else:
             self.wait_states = wait_states
diff --git a/taskflow/utils/misc.py b/taskflow/utils/misc.py
index 0bc0a054..c6b4a99c 100644
--- a/taskflow/utils/misc.py
+++ b/taskflow/utils/misc.py
@@ -396,7 +396,8 @@ class TransitionNotifier(object):
                 callback(state, *args, **kwargs)
             except Exception:
                 LOG.warn("Failure calling callback %s to notify about state"
-                         " transition %s", callback, state, exc_info=True)
+                         " transition %s, details: %s",
+                         callback, state, details, exc_info=True)
 
     def register(self, state, callback, args=None, kwargs=None):
         assert six.callable(callback), "Callback must be callable"