diff --git a/execution.py b/execution.py
index f79c3d35..9cebce92 100644
--- a/execution.py
+++ b/execution.py
@@ -103,7 +103,9 @@ def get_output_data(obj, input_data_all):
     return output, ui
 
 def format_value(x):
-    if isinstance(x, (int, float, bool, str)):
+    if x is None:
+        return None
+    elif isinstance(x, (int, float, bool, str)):
         return x
     else:
         return str(x)