diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index ff7c76a..af3f5ce 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -231,7 +231,8 @@ copies `kokoro-server` next to `libkokoro`; `build.rs` links against the library
 - `AppState` (`routes.rs`) keeps the engine behind a `tokio::sync::Mutex`. A request waits for the lock without a
   thread, then synthesizes in `spawn_blocking`, which also reads the thread-local `kokoro_last_error()`.
 - `POST /v1/audio/speech` parses the JSON body (`speech_request.rs`), then encodes the float samples as 16-bit PCM
-  WAV or raw PCM (`audio_encoding.rs`). Other formats get an OpenAI-style 400 error.
+  WAV or raw PCM (`audio_encoding.rs`). Other formats get an OpenAI-style 400 error. `input` and `voice` must not
+  contain a NUL byte (C strings would silently truncate it); such requests get a 400 naming the field.
 - `GET /v1/audio/voices` lists the loaded voices; `GET /health` answers `{"status":"ok"}`.
 - The listen socket (`listener.rs`) sets `SO_REUSEADDR` (`SO_EXCLUSIVEADDRUSE` on Windows) and never
   `SO_REUSEPORT`, so a second server on the same port fails instead of sharing it.
diff --git a/src/server/engine.rs b/src/server/engine.rs
index 83d4b15..90a1da9 100644
--- a/src/server/engine.rs
+++ b/src/server/engine.rs
@@ -108,9 +108,15 @@ impl Engine {
     }
 
     pub fn synthesize(&mut self, text: &str, voice: &str, speed: f32) -> Result<Audio, SynthesisError> {
-        // C strings end at the first NUL, as with the C++ server's c_str().
-        let c_prefix = |s: &str| CString::new(s.split('\0').next().unwrap_or_default()).expect("prefix has no NUL");
-        let (text, voice) = (c_prefix(text), c_prefix(voice));
+        // Reject rather than truncate: a NUL would silently end the string at the C boundary, losing the tail.
+        let c_arg = |text: &str| {
+            c_string(text).map_err(|message| SynthesisError {
+                status: kokoro_status::ERROR_INVALID_ARGUMENT,
+                message,
+            })
+        };
+        let text = c_arg(text)?;
+        let voice = c_arg(voice)?;
         let mut audio = Audio(kokoro_audio { samples: ptr::null_mut(), num_samples: 0, sample_rate: 0 });
         // SAFETY: self.ctx is live and exclusively borrowed; strings outlive the call; audio.0 is a valid out value.
         let status = unsafe { ffi::kokoro_synthesize(self.ctx, text.as_ptr(), voice.as_ptr(), speed, 0, &mut audio.0) };
diff --git a/src/server/speech_request.rs b/src/server/speech_request.rs
index 568f632..43ab03e 100644
--- a/src/server/speech_request.rs
+++ b/src/server/speech_request.rs
@@ -53,19 +53,30 @@ fn is_blank(text: &str) -> bool {
     text.bytes().all(|b| b == b' ' || (b'\t'..=b'\r').contains(&b))
 }
 
-fn parse_input(body: &Body) -> Result<String, RequestError> {
-    match optional_member(body, "input").and_then(Value::as_str) {
-        Some(text) if !is_blank(text) => Ok(text.to_owned()),
-        _ => fail("'input' must be a non-empty string", Some("input")),
+// C strings end at the first NUL byte, so a NUL would silently truncate the value at the engine boundary.
+fn check_no_nul(text: &str, field: &'static str) -> Result<(), RequestError> {
+    if text.contains('\0') {
+        fail(format!("'{}' must not contain a NUL byte", field), Some(field))
+    } else {
+        Ok(())
     }
 }
 
+fn parse_input(body: &Body) -> Result<String, RequestError> {
+    let text = match optional_member(body, "input").and_then(Value::as_str) {
+        Some(text) if !is_blank(text) => text,
+        _ => return fail("'input' must be a non-empty string", Some("input")),
+    };
+    check_no_nul(text, "input").map(|()| text.to_owned())
+}
+
 fn parse_voice(body: &Body, default_voice: &str) -> Result<String, RequestError> {
-    match optional_member(body, "voice") {
-        None => Ok(default_voice.to_owned()),
-        Some(Value::String(voice)) => Ok(voice.clone()),
-        Some(_) => fail("'voice' must be a string", Some("voice")),
-    }
+    let voice = match optional_member(body, "voice") {
+        None => return Ok(default_voice.to_owned()),
+        Some(Value::String(voice)) => voice,
+        Some(_) => return fail("'voice' must be a string", Some("voice")),
+    };
+    check_no_nul(voice, "voice").map(|()| voice.clone())
 }
 
 fn parse_speed(body: &Body) -> Result<f32, RequestError> {
@@ -138,6 +149,16 @@ mod tests {
         assert_eq!(request.format, ResponseFormat::Wav);
     }
 
+    #[test]
+    fn nul_bytes_are_rejected_instead_of_truncated() {
+        let input = rejected(r#"{"input":"Hello\u0000world"}"#);
+        assert_eq!(input.message, "'input' must not contain a NUL byte");
+        assert_eq!(input.param, Some("input"));
+        let voice = rejected(r#"{"input":"x","voice":"af_maple\u0000"}"#);
+        assert_eq!(voice.message, "'voice' must not contain a NUL byte");
+        assert_eq!(voice.param, Some("voice"));
+    }
+
     #[test]
     fn accepted_values() {
         let request =
diff --git a/src/server/tests/server_synthesis.rs b/src/server/tests/server_synthesis.rs
index 03ed160..8122923 100644
--- a/src/server/tests/server_synthesis.rs
+++ b/src/server/tests/server_synthesis.rs
@@ -74,6 +74,14 @@ fn server_synthesizes_audio() {
     assert_eq!((pcm.status, pcm.content_type.as_str()), (200, "audio/pcm"));
     assert!(!pcm.body.is_empty() && pcm.body.len() % 2 == 0);
 
+    // A NUL byte must produce a 400 naming the field, never 200 with truncated audio.
+    let nul_input = post(&agent, &url, r#"{"input":"Hello\u0000world"}"#);
+    assert_eq!(nul_input.status, 400);
+    assert_eq!(json(&nul_input)["error"]["param"], "input");
+    let nul_voice = post(&agent, &url, r#"{"input":"Hello world","voice":"af_maple\u0000"}"#);
+    assert_eq!(nul_voice.status, 400);
+    assert_eq!(json(&nul_voice)["error"]["param"], "voice");
+
     let voice = post(&agent, &url, r#"{"input":"Hello world","voice":"nope"}"#);
     assert_eq!(voice.status, 400);
     assert_eq!(json(&voice)["error"]["param"], "voice");