diff --git a/moto/swf/exceptions.py b/moto/swf/exceptions.py
index 4d0a9dede..d79281100 100644
--- a/moto/swf/exceptions.py
+++ b/moto/swf/exceptions.py
@@ -112,9 +112,9 @@ class SWFDecisionValidationException(SWFClientError):
         # prefix
         count = len(problems)
         if count < 2:
-            prefix = "{} validation error detected:"
+            prefix = "{} validation error detected: "
         else:
-            prefix = "{} validation errors detected:"
+            prefix = "{} validation errors detected: "
         super(SWFDecisionValidationException, self).__init__(
             prefix.format(count) + "; ".join(messages),
             "com.amazon.coral.validate#ValidationException"