diff --git a/src/machine/system_calls.rs b/src/machine/system_calls.rs
index 531f2436..aa70337e 100644
--- a/src/machine/system_calls.rs
+++ b/src/machine/system_calls.rs
@@ -4905,12 +4905,17 @@ impl Machine {
         let mut seen_set = IndexSet::new();
 
         {
+            let orig_heap_len = self.machine_st.heap.len();
             let mut iter = stackless_preorder_iter(&mut self.machine_st.heap, stored_v);
 
             while let Some(addr) = iter.next() {
                 let addr = unmark_cell_bits!(addr);
 
-                if addr.is_var() {
+                if addr.is_var() && addr.get_value() < orig_heap_len {
+                    // the length check is so we don't catalog
+                    // temporary variables created by the iterator,
+                    // that will be deleted when the iterator is
+                    // dropped, into seen_set.
                     seen_set.insert(addr);
                 }
             }