diff --git a/src/rust/protover/ffi.rs b/src/rust/protover/ffi.rs
index 404b158388..c86fe75227 100644
--- a/src/rust/protover/ffi.rs
+++ b/src/rust/protover/ffi.rs
@@ -190,7 +190,6 @@ pub extern "C" fn protover_is_supported_here(
 #[no_mangle]
 pub extern "C" fn protover_compute_for_old_tor(version: *const c_char) -> *const c_char {
     let supported: &'static CStr;
-    let elder_protocols: &'static [u8];
     let empty: &'static CStr;
 
     empty = empty_static_cstr();
@@ -208,19 +207,6 @@ pub extern "C" fn protover_compute_for_old_tor(version: *const c_char) -> *const
         Err(_) => return empty.as_ptr(),
     };
 
-    elder_protocols = compute_for_old_tor(&version);
-
-    // If we're going to pass it to C, there cannot be any intermediate NUL
-    // bytes.  An assert is okay here, since changing the const byte slice
-    // in protover.rs to contain a NUL byte somewhere in the middle would be a
-    // programming error.
-    assert!(byte_slice_is_c_like(elder_protocols));
-
-    // It's okay to unwrap the result of this function because
-    // we can see that the bytes we're passing into it 1) are valid UTF-8,
-    // 2) have no intermediate NUL bytes, and 3) are terminated with a NUL
-    // byte.
-    supported = CStr::from_bytes_with_nul(elder_protocols).unwrap_or(empty);
-
+    supported = compute_for_old_tor(&version);
     supported.as_ptr()
 }
diff --git a/src/rust/protover/protover.rs b/src/rust/protover/protover.rs
index d354d8e253..e9fcdddd9b 100644
--- a/src/rust/protover/protover.rs
+++ b/src/rust/protover/protover.rs
@@ -11,8 +11,6 @@ use std::collections::{HashMap, HashSet};
 use std::ops::Range;
 use std::string::String;
 
-use tor_util::strings::NUL_BYTE;
-
 /// The first version of Tor that included "proto" entries in its descriptors.
 /// Authorities should use this to decide whether to guess proto lines.
 ///
@@ -744,7 +742,7 @@ pub fn is_supported_here(proto: Proto, vers: Version) -> bool {
 ///
 /// # Returns
 ///
-/// A `&'static [u8]` encoding a list of protocol names and supported
+/// A `&'static CStr` encoding a list of protocol names and supported
 /// versions. The string takes the following format:
 ///
 /// "HSDir=1-1 LinkAuth=1"
@@ -753,27 +751,29 @@ pub fn is_supported_here(proto: Proto, vers: Version) -> bool {
 /// only for tor versions older than FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS.
 ///
 /// C_RUST_COUPLED: src/rust/protover.c `compute_for_old_tor`
-pub fn compute_for_old_tor(version: &str) -> &'static [u8] {
+pub fn compute_for_old_tor(version: &str) -> &'static CStr {
+    let empty: &'static CStr = cstr!("");
+
     if c_tor_version_as_new_as(version, FIRST_TOR_VERSION_TO_ADVERTISE_PROTOCOLS) {
-        return NUL_BYTE;
+        return empty;
     }
 
     if c_tor_version_as_new_as(version, "0.2.9.1-alpha") {
-        return b"Cons=1-2 Desc=1-2 DirCache=1 HSDir=1 HSIntro=3 HSRend=1-2 \
-                 Link=1-4 LinkAuth=1 Microdesc=1-2 Relay=1-2\0";
+        return cstr!("Cons=1-2 Desc=1-2 DirCache=1 HSDir=1 HSIntro=3 HSRend=1-2 \
+                      Link=1-4 LinkAuth=1 Microdesc=1-2 Relay=1-2");
     }
 
     if c_tor_version_as_new_as(version, "0.2.7.5") {
-        return b"Cons=1-2 Desc=1-2 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
-                 Link=1-4 LinkAuth=1 Microdesc=1-2 Relay=1-2\0";
+        return cstr!("Cons=1-2 Desc=1-2 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
+                      Link=1-4 LinkAuth=1 Microdesc=1-2 Relay=1-2");
     }
 
     if c_tor_version_as_new_as(version, "0.2.4.19") {
-        return b"Cons=1 Desc=1 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
-                 Link=1-4 LinkAuth=1 Microdesc=1 Relay=1-2\0";
+        return cstr!("Cons=1 Desc=1 DirCache=1 HSDir=1 HSIntro=3 HSRend=1 \
+                      Link=1-4 LinkAuth=1 Microdesc=1 Relay=1-2");
     }
 
-    NUL_BYTE
+    empty
 }
 
 #[cfg(test)]