diff --git a/redox-rt/src/sys.rs b/redox-rt/src/sys.rs
index 2a5110958e..a90cc926d0 100644
--- a/redox-rt/src/sys.rs
+++ b/redox-rt/src/sys.rs
@@ -624,7 +624,7 @@ pub fn open<T: AsRef<str>>(path: T, flags: usize) -> Result<usize> {
         let mut len = 0;
         buf[..prefix.len()].copy_from_slice(prefix);
         len += prefix.len();
-        let errno_bytes = format_errno(e.errno);
+        let errno_bytes = format_errno(e.errno as usize);
         buf[len..len + errno_bytes.len()].copy_from_slice(&errno_bytes);
         len += errno_bytes.len();
         buf[len] = b' ';