diff --git a/src/protocol/arena.rs b/src/protocol/arena.rs
index df2d5ce1cd2ce9d45e97f3d4260f835a6fe7009a..68dc8b5cd6f8ad83dbeef578b351f0d2055e557a 100644
--- a/src/protocol/arena.rs
+++ b/src/protocol/arena.rs
@@ -3,14 +3,16 @@ use core::hash::Hash;
 use core::marker::PhantomData;
 
 pub struct Id<T> {
-    pub(crate) index: u32,
+    // Not actually a signed index into the heap. But the index is set to -1 if
+    // we don't know an ID yet. This is checked during debug mode.
+    pub(crate) index: i32,
     _phantom: PhantomData<T>,
 }
 
 impl<T> Id<T> {
-    pub(crate) fn new(index: u32) -> Self {
-        Self{ index, _phantom: Default::default() }
-    }
+    pub(crate) fn new_invalid() -> Self     { Self{ index: -1, _phantom: Default::default() } }
+    pub(crate) fn new(index: i32) -> Self   { Self{ index, _phantom: Default::default() } }
+    pub(crate) fn is_invalid(&self) -> bool { self.index < 0 }
 }
 
 #[derive(Debug)]
@@ -46,15 +48,19 @@ impl<T> Arena<T> {
     pub fn new() -> Self {
         Self { store: vec![] }
     }
+
     pub fn alloc_with_id(&mut self, f: impl FnOnce(Id<T>) -> T) -> Id<T> {
-        use std::convert::TryFrom;
-        let id = Id::new(u32::try_from(self.store.len()).expect("Out of capacity!"));
+        // Lets keep this a runtime assert.
+        assert!(self.store.len() < i32::max_value() as usize, "Arena out of capacity");
+        let id = Id::new(self.store.len() as i32);
         self.store.push(f(id));
         id
     }
+
     pub fn iter(&self) -> impl Iterator<Item = &T> {
         self.store.iter()
     }
+
     pub fn len(&self) -> usize {
         self.store.len()
     }
@@ -62,11 +68,13 @@ impl<T> Arena<T> {
 impl<T> core::ops::Index<Id<T>> for Arena<T> {
     type Output = T;
     fn index(&self, id: Id<T>) -> &Self::Output {
+        debug_assert!(!id.is_invalid(), "attempted to index into Arena with an invalid id (index < 0)");
         self.store.index(id.index as usize)
     }
 }
 impl<T> core::ops::IndexMut<Id<T>> for Arena<T> {
     fn index_mut(&mut self, id: Id<T>) -> &mut Self::Output {
+        debug_assert!(!id.is_invalid(), "attempted to index_mut into Arena with an invalid id (index < 0)");
         self.store.index_mut(id.index as usize)
     }
 }
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