diff --git a/src/encoder.rs b/src/encoder.rs
index d7c7050..ceb4849 100644
--- a/src/encoder.rs
+++ b/src/encoder.rs
@@ -93,6 +93,7 @@ impl Default for Repeat {
 }
 
 /// Extension data.
+#[non_exhaustive]
 pub enum ExtensionData {
     /// Control extension. Use `ExtensionData::new_control_ext` to construct.
     Control {