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 {
diff --git a/tests/roundtrip.rs b/tests/roundtrip.rs
index 1baecf1..d8621da 100644
--- a/tests/roundtrip.rs
+++ b/tests/roundtrip.rs
@@ -1,6 +1,6 @@
 #![cfg(feature = "std")]
 
-use gif::{ColorOutput, Decoder, Encoder, Frame};
+use gif::{ColorOutput, Decoder, Encoder, Frame, AnyExtension};
 
 #[test]
 fn round_trip() {
@@ -68,6 +68,7 @@ fn round_trip_from_image(original: &[u8]) {
 
     let mut encoder = Encoder::new(vec![], width, height, &global_palette).unwrap();
     encoder.set_repeat(repeat).unwrap();
+    encoder.write_raw_extension(AnyExtension(gif::Extension::Comment as _), &[b"hello"]).unwrap();
     for frame in &frames {
         encoder.write_frame(frame).unwrap();
     }