diff --git a/src/types.rs b/src/types.rs
index ff053524..fd4a7736 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -212,14 +212,15 @@ impl ops::Add<u32> for Var {
     type Output = Var;
 
     fn add(self, rhs: u32) -> Self::Output {
-        Var {
-            idx: self.idx + rhs,
-        }
+        let idx = self.idx + rhs;
+        debug_assert!(idx <= Var::MAX_IDX);
+        Var { idx }
     }
 }
 
 impl ops::AddAssign<u32> for Var {
     fn add_assign(&mut self, rhs: u32) {
+        debug_assert!(self.idx + rhs <= Var::MAX_IDX);
         self.idx += rhs;
     }
 }