diff --git a/nac3core/src/typecheck/unification_table.rs b/nac3core/src/typecheck/unification_table.rs
index 19f836d8..619c23a7 100644
--- a/nac3core/src/typecheck/unification_table.rs
+++ b/nac3core/src/typecheck/unification_table.rs
@@ -1,5 +1,7 @@
 use std::rc::Rc;
 
+use itertools::izip;
+
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 pub struct UnificationKey(usize);
 
@@ -7,7 +9,7 @@ pub struct UnificationKey(usize);
 pub struct UnificationTable<V> {
     parents: Vec<usize>,
     ranks: Vec<u32>,
-    values: Vec<V>,
+    values: Vec<Option<V>>,
 }
 
 impl<V> UnificationTable<V> {
@@ -19,7 +21,7 @@ impl<V> UnificationTable<V> {
         let index = self.parents.len();
         self.parents.push(index);
         self.ranks.push(0);
-        self.values.push(v);
+        self.values.push(Some(v));
         UnificationKey(index)
     }
 
@@ -40,12 +42,12 @@ impl<V> UnificationTable<V> {
 
     pub fn probe_value(&mut self, a: UnificationKey) -> &V {
         let index = self.find(a);
-        &self.values[index]
+        self.values[index].as_ref().unwrap()
     }
 
     pub fn set_value(&mut self, a: UnificationKey, v: V) {
         let index = self.find(a);
-        self.values[index] = v;
+        self.values[index] = Some(v);
     }
 
     pub fn unioned(&mut self, a: UnificationKey, b: UnificationKey) -> bool {
@@ -77,12 +79,15 @@ where
     V: Clone,
 {
     pub fn get_send(&self) -> UnificationTable<V> {
-        let values = self.values.iter().map(|v| v.as_ref().clone()).collect();
+        let values = izip!(self.values.iter(), self.parents.iter())
+            .enumerate()
+            .map(|(i, (v, p))| if *p == i { v.as_ref().map(|v| v.as_ref().clone()) } else { None })
+            .collect();
         UnificationTable { parents: self.parents.clone(), ranks: self.ranks.clone(), values }
     }
 
     pub fn from_send(table: &UnificationTable<V>) -> UnificationTable<Rc<V>> {
-        let values = table.values.iter().cloned().map(Rc::new).collect();
+        let values = table.values.iter().cloned().map(|v| v.map(Rc::new)).collect();
         UnificationTable { parents: table.parents.clone(), ranks: table.ranks.clone(), values }
     }
 }