diff --git a/src/util_libc.rs b/src/util_libc.rs
index aae4637e..f64cc312 100644
--- a/src/util_libc.rs
+++ b/src/util_libc.rs
@@ -67,20 +67,21 @@ pub fn sys_fill_exact(
     mut buf: &mut [MaybeUninit<u8>],
     sys_fill: impl Fn(&mut [MaybeUninit<u8>]) -> libc::ssize_t,
 ) -> Result<(), Error> {
-    use core::cmp::Ordering::*;
-
     while !buf.is_empty() {
         let res = sys_fill(buf);
-        match res.cmp(&0) {
-            Greater => buf = buf.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?,
-            Less => {
+        match res {
+            res if res > 0 => buf = buf.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?,
+            -1 => {
                 let err = last_os_error();
                 // We should try again if the call was interrupted.
                 if err.raw_os_error() != Some(libc::EINTR) {
                     return Err(err);
                 }
             }
-            Equal => return Err(Error::UNEXPECTED),
+            // Negative return codes not equal to -1 should be impossible.
+            // EOF (ret = 0) should be impossible, as the data we are reading
+            // should be an infinite stream of random bytes.
+            _ => return Err(Error::UNEXPECTED),
         }
     }
     Ok(())