diff --git a/wutil-rs/src/string.rs b/wutil-rs/src/string.rs
index 92492e25..892f3f3f 100644
--- a/wutil-rs/src/string.rs
+++ b/wutil-rs/src/string.rs
@@ -20,18 +20,22 @@ pub unsafe extern "C" fn wstrdup(s: *const c_char) -> *mut c_char {
     alloc_string(unsafe { CStr::from_ptr(s) })
 }
 
-/// Returns a `wmalloc`-managed C-style string of the first `n` bytes of
-/// `s`, which cannot be null.
+/// Returns a `wmalloc`-managed C-style string of the first `len` bytes of `s`,
+/// which cannot be null.
 ///
-/// If `n` exceeds the length of `s`, uses the lesser value.
+/// If `len` exceeds the length of `s`, uses the lesser value.
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn wstrndup(s: *const c_char, len: usize) -> *mut c_char {
     assert!(!s.is_null());
-    let s = unsafe { CStr::from_ptr(s) };
-    let len = usize::min(len, s.count_bytes()) + 1;
+    let len = unsafe {
+        slice::from_raw_parts(s, len)
+            .into_iter()
+            .position(|p| *p == 0)
+            .unwrap_or(len)
+    };
     let copy: *mut c_char = alloc_bytes(len + 1).cast(); // Implicitly zeroed.
     unsafe {
-        ptr::copy_nonoverlapping(s.as_ptr(), copy, len);
+        ptr::copy_nonoverlapping(s, copy, len);
     }
     copy.cast::<c_char>()
 }