diff --git a/fathom/src/env.rs b/fathom/src/env.rs
index 62d12934c..07039deae 100644
--- a/fathom/src/env.rs
+++ b/fathom/src/env.rs
@@ -17,7 +17,8 @@
 //! and [`SliceEnv`], but when we need to copy environments often, we use a
 //! [`SharedEnv`] to increase the amount of sharing at the expense of locality.
 
-use std::{fmt, ops::Add};
+use std::fmt;
+use std::ops::Add;
 
 /// Underlying variable representation.
 type RawVar = u16;
@@ -133,6 +134,13 @@ pub fn levels() -> impl Iterator<Item = Level> {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct EnvLen(RawVar);
 
+impl Add<Index> for EnvLen {
+    type Output = Self;
+    fn add(self, rhs: Index) -> Self::Output {
+        Self(self.0 + rhs.0) // FIXME: check overflow?
+    }
+}
+
 impl EnvLen {
     /// Construct a new, empty environment.
     pub fn new() -> EnvLen {