diff --git a/src/machine/mock_wam.rs b/src/machine/mock_wam.rs
index d157ee14..0fcebd41 100644
--- a/src/machine/mock_wam.rs
+++ b/src/machine/mock_wam.rs
@@ -56,7 +56,7 @@ impl MockWAM {
         let mut printer = HCPrinter::new(
             &mut self.machine_st.heap,
             &mut self.machine_st.stack,
-            &mut self.machine_st.arena,
+            &self.machine_st.arena,
             &self.op_dir,
             PrinterOutputter::new(),
             term_write_result.heap_loc,