diff --git a/docs/utils.md b/docs/utils.md
index 91a438bf..0a4482f0 100644
--- a/docs/utils.md
+++ b/docs/utils.md
@@ -149,7 +149,7 @@ Parameters:
 Attributes:
 
     string              (str)   the input datetime string
-    format              (int)   the format rule that was used to decode the datetime string
+    format              (int)   the format rule that was used to decode the datetime string. None if conversion fails
     naive               (int)   timestamp based on locally configured timezone. None if conversion fails
     utc                 (int)   aware timestamp only if UTC timezone detected in datetime string. None if conversion fails
 
diff --git a/jc/utils.py b/jc/utils.py
index bbf135ce..f345a3bf 100644
--- a/jc/utils.py
+++ b/jc/utils.py
@@ -246,7 +246,7 @@ class timestamp:
     Attributes:
 
         string              (str)   the input datetime string
-        format              (int)   the format rule that was used to decode the datetime string
+        format              (int)   the format rule that was used to decode the datetime string. None if conversion fails
         naive               (int)   timestamp based on locally configured timezone. None if conversion fails
         utc                 (int)   aware timestamp only if UTC timezone detected in datetime string. None if conversion fails
     """