diff --git a/stix2/exceptions.py b/stix2/exceptions.py
index 1165ecf..3043047 100644
--- a/stix2/exceptions.py
+++ b/stix2/exceptions.py
@@ -53,7 +53,7 @@ class ImmutableError(STIXError, ValueError):
         self.key = key
 
     def __str__(self):
-        msg = "Cannot modify '{0.key}' property in {0.cls} after creation."
+        msg = "Cannot modify '{0.key}' property in '{0.cls.__name__}' after creation."
         return msg.format(self)