diff --git a/crates/environ/src/collections/entity_set.rs b/crates/environ/src/collections/entity_set.rs
index 5eac9c24383f..d4bc95647c0e 100644
--- a/crates/environ/src/collections/entity_set.rs
+++ b/crates/environ/src/collections/entity_set.rs
@@ -67,7 +67,7 @@ where
     /// Returns `true` if `k` was not present in the set, i.e. this is a
     /// newly-added element. Returns `false` otherwise.
     pub fn insert(&mut self, k: K) -> Result<bool, OutOfMemory> {
-        self.inner.try_ensure_capacity(k.index())?;
+        self.inner.try_ensure_capacity(k.index() + 1)?;
         Ok(self.inner.insert(k))
     }
 
diff --git a/crates/fuzzing/tests/oom.rs b/crates/fuzzing/tests/oom.rs
index 2e022e5aefd8..3e55ca6cc511 100644
--- a/crates/fuzzing/tests/oom.rs
+++ b/crates/fuzzing/tests/oom.rs
@@ -124,6 +124,24 @@ fn secondary_map_try_insert() -> Result<()> {
     })
 }
 
+#[test]
+fn entity_set_ensure_capacity() -> Result<()> {
+    OomTest::new().test(|| {
+        let mut set = EntitySet::<Key>::new();
+        set.ensure_capacity(100)?;
+        Ok(())
+    })
+}
+
+#[test]
+fn entity_set_insert() -> Result<()> {
+    OomTest::new().test(|| {
+        let mut set = EntitySet::<Key>::new();
+        set.insert(Key::from_u32(256))?;
+        Ok(())
+    })
+}
+
 #[test]
 fn vec_with_capacity() -> Result<()> {
     OomTest::new().test(|| {