diff --git a/Cargo.toml b/Cargo.toml
index d3c3655..8866016 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -32,6 +32,9 @@ serde_json = { version = "1", optional = true }
 
 [dev-dependencies]
 axum = { version = "0.7", default-features = false }
+axum-test = "14"
+tokio = { version = "1", features = ["full"] }
+tokio-test = "0.4"
 
 [package.metadata.docs.rs]
 all-features = true
diff --git a/README.md b/README.md
index ba911b6..a7a3ee3 100644
--- a/README.md
+++ b/README.md
@@ -219,6 +219,12 @@ Contributions are always welcome! If you have an idea for a feature or find a
 bug, let me know. PR's are appreciated, but if it's not a small change, please
 open an issue first so we're all on the same page!
 
+### Testing
+
+```sh
+cargo +nightly test --all-features
+```
+
 ## License
 
 `axum-htmx` is dual-licensed under either
diff --git a/src/error.rs b/src/error.rs
index d89d096..1c2f782 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -1,11 +1,15 @@
 use std::{error, fmt};
 
 use axum_core::response::IntoResponse;
-use http::{header::InvalidHeaderValue, StatusCode};
+use http::{
+    header::{InvalidHeaderValue, MaxSizeReached},
+    StatusCode,
+};
 
 #[derive(Debug)]
 pub enum HxError {
     InvalidHeaderValue(InvalidHeaderValue),
+    TooManyResponseHeaders(MaxSizeReached),
 
     #[cfg(feature = "serde")]
     #[cfg_attr(feature = "unstable", doc(cfg(feature = "serde")))]
@@ -18,6 +22,12 @@ impl From<InvalidHeaderValue> for HxError {
     }
 }
 
+impl From<MaxSizeReached> for HxError {
+    fn from(value: MaxSizeReached) -> Self {
+        Self::TooManyResponseHeaders(value)
+    }
+}
+
 #[cfg(feature = "serde")]
 #[cfg_attr(feature = "unstable", doc(cfg(feature = "serde")))]
 impl From<serde_json::Error> for HxError {
@@ -30,6 +40,7 @@ impl fmt::Display for HxError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             HxError::InvalidHeaderValue(err) => write!(f, "Invalid header value: {err}"),
+            HxError::TooManyResponseHeaders(err) => write!(f, "Too many response headers: {err}"),
             #[cfg(feature = "serde")]
             HxError::Json(err) => write!(f, "Json: {err}"),
         }
diff --git a/src/responders/vary.rs b/src/responders/vary.rs
index 1ee7312..03866da 100644
--- a/src/responders/vary.rs
+++ b/src/responders/vary.rs
@@ -1,8 +1,13 @@
 use axum_core::response::{IntoResponseParts, ResponseParts};
-use http::header::VARY;
+use http::header::{HeaderValue, VARY};
 
 use crate::{extractors, headers, HxError};
 
+const HX_REQUEST: HeaderValue = HeaderValue::from_static(headers::HX_REQUEST);
+const HX_TARGET: HeaderValue = HeaderValue::from_static(headers::HX_TARGET);
+const HX_TRIGGER: HeaderValue = HeaderValue::from_static(headers::HX_TRIGGER);
+const HX_TRIGGER_NAME: HeaderValue = HeaderValue::from_static(headers::HX_TRIGGER_NAME);
+
 /// The `Vary: HX-Request` header.
 ///
 /// You may want to add this header to the response if your handler responds differently based on
@@ -21,8 +26,7 @@ impl IntoResponseParts for VaryHxRequest {
     type Error = HxError;
 
     fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
-        res.headers_mut()
-            .insert(VARY, headers::HX_REQUEST.try_into()?);
+        res.headers_mut().try_append(VARY, HX_REQUEST)?;
 
         Ok(res)
     }
@@ -50,8 +54,7 @@ impl IntoResponseParts for VaryHxTarget {
     type Error = HxError;
 
     fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
-        res.headers_mut()
-            .insert(VARY, headers::HX_TARGET.try_into()?);
+        res.headers_mut().try_append(VARY, HX_TARGET)?;
 
         Ok(res)
     }
@@ -79,8 +82,7 @@ impl IntoResponseParts for VaryHxTrigger {
     type Error = HxError;
 
     fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
-        res.headers_mut()
-            .insert(VARY, headers::HX_TRIGGER.try_into()?);
+        res.headers_mut().try_append(VARY, HX_TRIGGER)?;
 
         Ok(res)
     }
@@ -108,8 +110,7 @@ impl IntoResponseParts for VaryHxTriggerName {
     type Error = HxError;
 
     fn into_response_parts(self, mut res: ResponseParts) -> Result<ResponseParts, Self::Error> {
-        res.headers_mut()
-            .insert(VARY, headers::HX_TRIGGER_NAME.try_into()?);
+        res.headers_mut().try_append(VARY, HX_TRIGGER_NAME)?;
 
         Ok(res)
     }
@@ -121,3 +122,20 @@ impl extractors::HxTriggerName {
         VaryHxTriggerName
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use axum::{routing::get, Router};
+    use std::collections::hash_set::HashSet;
+
+    #[tokio::test]
+    async fn multiple_headers() {
+        let app = Router::new().route("/", get(|| async { (VaryHxRequest, VaryHxTarget, "foo") }));
+        let server = axum_test::TestServer::new(app).unwrap();
+
+        let resp = server.get("/").await;
+        let values: HashSet<HeaderValue> = resp.iter_headers_by_name("vary").cloned().collect();
+        assert_eq!(values, HashSet::from([HX_REQUEST, HX_TARGET]));
+    }
+}