diff --git a/lib/esse/document.rb b/lib/esse/document.rb
index bb663d5..e3ee53c 100644
--- a/lib/esse/document.rb
+++ b/lib/esse/document.rb
@@ -109,10 +109,10 @@ def document_for_partial_update(source)
     end
 
     def inspect
-      attributes = %i[id routing source].map do |attr|
-        value = send(attr)
+      attributes = {id: :id, routing: :routing, source: :memoized_source}.map do |attr_name, attr_src|
+        value = send(attr_src)
         next unless value
-        "#{attr}: #{value.inspect}"
+        "#{attr_name}: #{value.inspect}"
       rescue
         nil
       end.compact.join(', ')
@@ -131,9 +131,17 @@ def mutations
     end
 
     def mutated_source
-      return source unless @__mutations__
+      return memoized_source unless @__mutations__
 
-      @__mutated_source__ ||= source.merge(@__mutations__)
+      @__mutated_source__ ||= memoized_source.merge(@__mutations__)
+    end
+
+    protected
+
+    def memoized_source
+      return @__memoized_source__ if defined?(@__memoized_source__)
+
+      @__memoized_source__ = source || {}
     end
   end
 end
diff --git a/spec/esse/document_spec.rb b/spec/esse/document_spec.rb
index af0da6b..18f9609 100644
--- a/spec/esse/document_spec.rb
+++ b/spec/esse/document_spec.rb
@@ -133,6 +133,23 @@ def source
         expect(document.to_bulk(data: true)).to eq(_id: 1, _type: 'foo', routing: 'bar', timeout: 10, data: {})
       end
     end
+
+    context 'when the document source is not memoized' do
+      before do
+        document_class.class_eval do
+          def source
+            { foo: SecureRandom.hex }
+          end
+        end
+      end
+
+      it 'memoizes the source' do
+        data1 = document.to_bulk(data: true)[:data]
+        expect(document.to_bulk(data: true)[:data]).to eq(data1)
+        data2 = document.to_bulk(data: true)[:data]
+        expect(data1).to be(data2)
+      end
+    end
   end
 
   describe '#doc_header' do