diff --git a/src/prolog/machine/system_calls.rs b/src/prolog/machine/system_calls.rs
index 17d456ed..c71fc5ba 100644
--- a/src/prolog/machine/system_calls.rs
+++ b/src/prolog/machine/system_calls.rs
@@ -5266,7 +5266,10 @@ impl MachineState {
                              let salt = hkdf::Salt::new(digest_alg, &salt);
                              let mut bytes : Vec<u8> = Vec::new();
                              bytes.resize(length, 0);
-                             salt.extract(&data).expand(&[&info[..]], MyKey(length)).unwrap().fill(&mut bytes).unwrap();
+                             match salt.extract(&data).expand(&[&info[..]], MyKey(length)) {
+                                 Ok(r) => { r.fill(&mut bytes).unwrap(); }
+                                 _ => { self.fail = true; return Ok(()); }
+                             }
 
                              Addr::HeapCell(self.heap.to_list(bytes.iter().map(|b| HeapCellValue::Integer(Rc::new(Integer::from(*b))))))
                         };