diff --git a/lib/galaxy/tools/__init__.py b/lib/galaxy/tools/__init__.py
index 957dcf4fbba..a8ccec0b37a 100644
--- a/lib/galaxy/tools/__init__.py
+++ b/lib/galaxy/tools/__init__.py
@@ -2430,7 +2430,7 @@ class Tool(Dictifiable):
 
         # load job parameters into incoming
         tool_message = ""
-        tool_warnings = ""
+        tool_warnings = None
         if job:
             try:
                 job_params = job.get_param_values(self.app, ignore_errors=True)