diff --git a/src/liballoc/btree/map.rs b/src/liballoc/btree/map.rs
index b114dc640fbaf..b320bed54320a 100644
--- a/src/liballoc/btree/map.rs
+++ b/src/liballoc/btree/map.rs
@@ -1748,6 +1748,11 @@ impl<'a, K: Ord, Q: ?Sized, V> Index<&'a Q> for BTreeMap<K, V>
 {
     type Output = V;
 
+    /// Returns a reference to the value corresponding to the supplied key.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the key is not present in the `BTreeMap`.
     #[inline]
     fn index(&self, key: &Q) -> &V {
         self.get(key).expect("no entry found for key")
diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index b01420f36a0c3..82a687ae5e493 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -1384,9 +1384,14 @@ impl<'a, K, Q: ?Sized, V, S> Index<&'a Q> for HashMap<K, V, S>
 {
     type Output = V;
 
+    /// Returns a reference to the value corresponding to the supplied key.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the key is not present in the `HashMap`.
     #[inline]
-    fn index(&self, index: &Q) -> &V {
-        self.get(index).expect("no entry found for key")
+    fn index(&self, key: &Q) -> &V {
+        self.get(key).expect("no entry found for key")
     }
 }