diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 0537057666175..fa9f2131c0c1d 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -1379,9 +1379,6 @@ impl<T, A: Allocator> Vec<T, A> {
         }
 
         let len = self.len();
-        if index > len {
-            assert_failed(index, len);
-        }
 
         // space for the new element
         if len == self.buf.capacity() {
@@ -1393,9 +1390,15 @@ impl<T, A: Allocator> Vec<T, A> {
             // The spot to put the new value
             {
                 let p = self.as_mut_ptr().add(index);
-                // Shift everything over to make space. (Duplicating the
-                // `index`th element into two consecutive places.)
-                ptr::copy(p, p.offset(1), len - index);
+                if index < len {
+                    // Shift everything over to make space. (Duplicating the
+                    // `index`th element into two consecutive places.)
+                    ptr::copy(p, p.offset(1), len - index);
+                } else if index == len {
+                    // No elements need shifting.
+                } else {
+                    assert_failed(index, len);
+                }
                 // Write it in, overwriting the first copy of the `index`th
                 // element.
                 ptr::write(p, element);