diff --git a/src/rstd/atomic/au64.rs b/src/rstd/atomic/au64.rs
index f7955dc..d6222c0 100644
--- a/src/rstd/atomic/au64.rs
+++ b/src/rstd/atomic/au64.rs
@@ -78,4 +78,12 @@ mod tests {
     fn cannot_parse_empty_slice() {
         assert_eq!(u64::parse_slice(&[]).unwrap_err(), IntParseError::Eof);
     }
+
+    #[test]
+    fn cannot_parse_oversized_slice() {
+        assert_eq!(
+            u64::parse_slice(&[0, 0, 0, 0, 0, 0, 0, 0, 0]).unwrap_err(),
+            IntParseError::ExtraData(1),
+        );
+    }
 }