diff --git a/lib/mongo/operation/shared/idable.rb b/lib/mongo/operation/shared/idable.rb
index eaf570b042..82fc3fe5b1 100644
--- a/lib/mongo/operation/shared/idable.rb
+++ b/lib/mongo/operation/shared/idable.rb
@@ -58,7 +58,13 @@ def has_id?(doc)
       def ensure_ids(documents)
         @ids = []
         documents.collect do |doc|
-          doc_with_id = has_id?(doc) ? doc : doc.merge(_id: id_generator.generate)
+          doc_with_id = if has_id?(doc)
+                          doc
+                        else
+                          doc.delete('_id')
+                          doc.merge(_id: id_generator.generate)
+                        end
+
           @ids << id(doc_with_id)
           doc_with_id
         end
diff --git a/spec/mongo/operation/insert_spec.rb b/spec/mongo/operation/insert_spec.rb
index 2c15d0fef6..3393c5211c 100644
--- a/spec/mongo/operation/insert_spec.rb
+++ b/spec/mongo/operation/insert_spec.rb
@@ -100,6 +100,25 @@
         expect(inserted_ids).to eq(collection_ids)
       end
     end
+
+    context 'when document contains a nil id' do
+
+      let(:documents) do
+        [{ 'field' => 'test', '_id' => nil }]
+      end
+
+      let(:inserted_ids) do
+        insert.execute(authorized_primary, context: context).inserted_ids
+      end
+
+      let(:collection_ids) do
+        authorized_collection.find(field: 'test').collect { |d| d['_id'] }
+      end
+
+      it 'adds an id to the documents' do
+        expect(inserted_ids).to eq(collection_ids)
+      end
+    end
   end
 
   describe '#execute' do