diff --git a/lib/travis/api/v3/extensions/belongs_to.rb b/lib/travis/api/v3/extensions/belongs_to.rb
index 1fec8a7e..28aaa918 100644
--- a/lib/travis/api/v3/extensions/belongs_to.rb
+++ b/lib/travis/api/v3/extensions/belongs_to.rb
@@ -41,7 +41,7 @@ module Travis::API::V3
       end
 
       def []=(key, value)
-        value &&= value.sub("#{self.class.parent}::") if self.class.polymorfic_foreign_types.include?(key)
+        value &&= value.sub("#{self.class.parent}::", ''.freeze) if self.class.polymorfic_foreign_types.include?(key)
         super(key, value)
       end
     end
diff --git a/spec/v3/extensions/belongs_to_spec.rb b/spec/v3/extensions/belongs_to_spec.rb
new file mode 100644
index 00000000..dd031ab5
--- /dev/null
+++ b/spec/v3/extensions/belongs_to_spec.rb
@@ -0,0 +1,17 @@
+require 'spec_helper'
+
+describe Travis::API::V3::Extensions::BelongsTo do
+  describe 'reading polymorphic relation' do
+    subject(:repo) { Travis::API::V3::Models::Repository.first }
+    example { expect(repo.owner).to be_a(Travis::API::V3::Models::User) }
+  end
+
+  describe 'writing polymorphic relation' do
+    let(:repo) { Travis::API::V3::Models::Repository.create(owner: user) }
+    let(:user) { Travis::API::V3::Models::User.create }
+    after      { repo.destroy; user.destroy }
+
+    example { expect(repo.owner).to be_a(Travis::API::V3::Models::User) }
+    example { expect(::Repository.find(repo.id).owner).to be_a(::User)  }
+  end
+end
\ No newline at end of file