diff --git a/src/ethernetd/scheme.rs b/src/ethernetd/scheme.rs
index bbd6bc94df..f6da2aa7a3 100644
--- a/src/ethernetd/scheme.rs
+++ b/src/ethernetd/scheme.rs
@@ -85,7 +85,11 @@ impl SchemeMut for EthernetScheme {
         }
     }
 
-    fn dup(&mut self, id: usize, _buf: &[u8]) -> Result<usize> {
+    fn dup(&mut self, id: usize, buf: &[u8]) -> Result<usize> {
+        if ! buf.is_empty() {
+            return Err(Error::new(EINVAL));
+        }
+
         let next_id = self.next_id;
         self.next_id += 1;
 
diff --git a/src/ipd/main.rs b/src/ipd/main.rs
index 49e332fcf7..c5ce140634 100644
--- a/src/ipd/main.rs
+++ b/src/ipd/main.rs
@@ -150,7 +150,11 @@ impl SchemeMut for Ipd {
         }
     }
 
-    fn dup(&mut self, file: usize, _buf: &[u8]) -> Result<usize> {
+    fn dup(&mut self, file: usize, buf: &[u8]) -> Result<usize> {
+        if ! buf.is_empty() {
+            return Err(Error::new(EINVAL));
+        }
+
         let handle = {
             let handle = self.handles.get(&file).ok_or(Error::new(EBADF))?;
             Handle {