diff --git a/src/style.rs b/src/style.rs
index ec6496c7..176e5b98 100644
--- a/src/style.rs
+++ b/src/style.rs
@@ -145,6 +145,8 @@ impl ProgressStyle {
     }
 
     /// Adds a custom key that references a `&ProgressState` to the template
+    ///
+    /// Note that custom keys cannot override built-in template keys.
     pub fn with_key(mut self, key: &'static str, f: Format) -> ProgressStyle {
         self.format_map.0.insert(key, f);
         self