diff --git a/src/machine/machine_state_impl.rs b/src/machine/machine_state_impl.rs
index 931f0f11..85fac031 100644
--- a/src/machine/machine_state_impl.rs
+++ b/src/machine/machine_state_impl.rs
@@ -544,6 +544,9 @@ impl MachineState {
             (HeapCellValueTag::Atom, (name, arity)) => {
                 self.fail = !(arity == 0 && name == atom);
             }
+            (HeapCellValueTag::CStr, cstr_atom) if atom == atom!("[]") => {
+                self.fail = cstr_atom != atom!("");
+            }
             (HeapCellValueTag::Char, c1) => {
                 if let Some(c2) = atom.as_char() {
                     self.fail = c1 != c2;
diff --git a/src/machine/system_calls.rs b/src/machine/system_calls.rs
index becc644e..a407d7fa 100644
--- a/src/machine/system_calls.rs
+++ b/src/machine/system_calls.rs
@@ -2494,9 +2494,10 @@ impl Machine {
             }
         };
 
+        let output = self.machine_st.store(self.machine_st.deref(self.machine_st.registers[3]));
         let atom = self.machine_st.atom_tbl.build_with(&string);
-        self.machine_st.unify_complete_string(atom, self.machine_st.store(self.machine_st.deref(self.machine_st.registers[3])));
 
+        self.machine_st.unify_complete_string(atom, output);
         Ok(())
     }