diff --git a/lib/droplet_kit/resources/project_resource.rb b/lib/droplet_kit/resources/project_resource.rb
index 32fc4af1..3b98fcec 100644
--- a/lib/droplet_kit/resources/project_resource.rb
+++ b/lib/droplet_kit/resources/project_resource.rb
@@ -49,7 +49,7 @@ def to_urn(resources)
           resources.to_a.map do |resource|
             if resource.is_a?(String) && DropletKit::BaseModel.valid_urn?(resource)
               resource
-            elsif resource.try(:urn) && DropletKit::BaseModel.valid_urn?(resource.urn)
+            elsif resource.respond_to?(:urn) && resource.urn && DropletKit::BaseModel.valid_urn?(resource.urn)
               resource.urn
             else
               raise DropletKit::Error.new("cannot assign resource without valid urn: #{resource}")
diff --git a/spec/lib/droplet_kit/resources/project_resource_spec.rb b/spec/lib/droplet_kit/resources/project_resource_spec.rb
index a2683d6c..7d8fdcf8 100644
--- a/spec/lib/droplet_kit/resources/project_resource_spec.rb
+++ b/spec/lib/droplet_kit/resources/project_resource_spec.rb
@@ -127,4 +127,28 @@
       let(:api_path) { '/v2/projects/c177dc8c-12c1-4483-af1c-877eed0f14cb/resources' }
     end
   end
+
+  describe '#assign_resources' do
+    it 'calls the api when passing a valid urn' do
+      request = stub_do_api('/v2/projects/:id/resources', :post)
+        .to_return(body: '', status: 201)
+
+      resource.assign_resources(['do:droplet:123'])
+      expect(request).to have_been_made
+    end
+
+    it 'calls the api when passing a model than can be converted to urn' do
+      request = stub_do_api('/v2/projects/:id/resources', :post)
+        .to_return(body: '', status: 201)
+
+      resource.assign_resources([DropletKit::Droplet.new(id: 123)])
+      expect(request).to have_been_made
+    end
+
+    it 'raises an error when the resouce cannot be assigned' do
+      expect {
+        resource.assign_resources(['invalid'])
+      }.to raise_error(DropletKit::Error, 'cannot assign resource without valid urn: invalid')
+    end
+  end
 end