diff --git a/src/sources/fluent/mod.rs b/src/sources/fluent/mod.rs
index 76e51ae7a066d..f25f413590324 100644
--- a/src/sources/fluent/mod.rs
+++ b/src/sources/fluent/mod.rs
@@ -1,3 +1,4 @@
+use std::collections::HashMap;
 use std::io::{self, Read};
 use std::net::SocketAddr;
 use std::time::Duration;
@@ -9,8 +10,8 @@ use codecs::{BytesDeserializerConfig, StreamDecodingError};
 use flate2::read::MultiGzDecoder;
 use lookup::lookup_v2::parse_value_path;
 use lookup::{metadata_path, owned_value_path, path, OwnedValuePath, PathPrefix};
-use rmp_serde::{decode, Deserializer};
-use serde::Deserialize;
+use rmp_serde::{decode, Deserializer, Serializer};
+use serde::{Deserialize, Serialize};
 use smallvec::{smallvec, SmallVec};
 use tokio_util::codec::Decoder;
 use vector_config::configurable_component;
@@ -532,15 +533,18 @@ impl TcpSourceAcker for FluentAcker {
             return None;
         }
 
-        let mut acks = String::new();
+        let mut buf = Vec::new();
+        let mut ser = Serializer::new(&mut buf);
+        let mut ack_map = HashMap::new();
+
         for chunk in self.chunks {
-            let ack = match ack {
-                TcpSourceAck::Ack => format!(r#"{{"ack": "{}"}}"#, chunk),
-                _ => String::from("{}"),
+            ack_map.clear();
+            if let TcpSourceAck::Ack = ack {
+                ack_map.insert("ack", chunk);
             };
-            acks.push_str(&ack);
+            ack_map.serialize(&mut ser).unwrap();
         }
-        Some(acks.into())
+        Some(buf.into())
     }
 }
 
@@ -861,7 +865,8 @@ mod tests {
     async fn ack_delivered_with_chunk() {
         let (result, output) = check_acknowledgements(EventStatus::Delivered, true).await;
         assert_eq!(result.unwrap().unwrap(), output.len());
-        assert!(output.starts_with(b"{\"ack\":"));
+        let expected: Vec<u8> = vec![0x81, 0xa3, 0x61, 0x63]; // { "ack": ...
+        assert_eq!(output[..expected.len()], expected);
     }
 
     #[tokio::test]
@@ -875,7 +880,8 @@ mod tests {
     async fn ack_failed_with_chunk() {
         let (result, output) = check_acknowledgements(EventStatus::Rejected, true).await;
         assert_eq!(result.unwrap().unwrap(), output.len());
-        assert_eq!(output, &b"{}"[..]);
+        let expected: Vec<u8> = vec![0x80]; // { }
+        assert_eq!(output, expected);
     }
 
     async fn check_acknowledgements(