diff --git a/src/transaction/message.rs b/src/transaction/message.rs
index e9379e0e..32da9da0 100644
--- a/src/transaction/message.rs
+++ b/src/transaction/message.rs
@@ -2,7 +2,7 @@ use super::{endpoint::EndpointInner, make_call_id};
 use crate::{rsip_ext::extract_uri_from_contact, transaction::make_via_branch, Result};
 use rsip::{
     header,
-    headers::Route,
+    headers::{ContentLength, Route},
     prelude::{HeadersExt, ToTypedHeader, UntypedHeader},
     Error, Header, Request, Response, StatusCode,
 };
@@ -312,6 +312,7 @@ impl EndpointInner {
                 cseq.mut_method(rsip::Method::Ack).ok();
             }
         });
+        headers.push(Header::ContentLength(ContentLength::default())); // 0 because of vec![] below
         headers.unique_push(Header::UserAgent(self.user_agent.clone().into()));
         Ok(rsip::Request {
             method: rsip::Method::Ack,
diff --git a/src/transaction/tests/test_client.rs b/src/transaction/tests/test_client.rs
index efb14e50..e64a4e48 100644
--- a/src/transaction/tests/test_client.rs
+++ b/src/transaction/tests/test_client.rs
@@ -133,6 +133,18 @@ Content-Length: 0\r\n\r\n";
     let expected_uri = Uri::try_from("sip:uas@192.0.2.55:5080;transport=tcp")?;
     assert_eq!(ack.uri, expected_uri, "ACK must target the remote Contact");
 
+    // Check Content-Length
+    let content_length: String = ack
+        .headers
+        .iter()
+        .filter_map(|header| match header {
+            Header::ContentLength(content_length) => Some(content_length.value().to_string()),
+            _ => None,
+        })
+        .next()
+        .expect("ACK must include a Content-Length header");
+
+    assert_eq!(content_length, "0", "Content-Length of ACK must be 0");
     let routes: Vec<String> = ack
         .headers
         .iter()