diff --git a/public/amtl/am-vector.h b/public/amtl/am-vector.h
index 2b0b8feb..4f01bb97 100644
--- a/public/amtl/am-vector.h
+++ b/public/amtl/am-vector.h
@@ -167,6 +167,14 @@ class Vector : public AllocPolicy
     return growIfNeeded(desired - length());
   }
 
+  Vector &operator =(Moveable<Vector<T, AllocPolicy> > other) {
+    data_ = other->data_;
+    nitems_ = other->nitems_;
+    maxsize_ = other->maxsize_;
+    other->reset();
+    return *this;
+  }
+
  private:
   // These are disallowed because they basically violate the failure handling
   // model for AllocPolicies and are also likely to have abysmal performance.
@@ -186,10 +194,15 @@ class Vector : public AllocPolicy
   }
 
   bool moveUp(size_t at) {
-    assert(at < nitems_);
-    if (!append(Moveable<T>(data_[nitems_ - 1])))
+    // Note: we don't use append() here. Passing an element as a Moveable into
+    // insert() or append() can break, since the underlying storage could be
+    // reallocated, invalidating the Moveable reference. Instead, we inline
+    // the logic to append() to ensure growIfNeeded occurs before any
+    // references are taken.
+    if (!growIfNeeded(1))
       return false;
-
+    new (&data_[nitems_]) T(Moveable<T>(data_[nitems_ - 1]));
+    nitems_++;
     for (size_t i = nitems_ - 2; i > at; i--)
       data_[i] = Moveable<T>(data_[i - 1]);
     return true;