diff --git a/lib/cocoaseeds/core.rb b/lib/cocoaseeds/core.rb
index e6d2081..c22d07e 100644
--- a/lib/cocoaseeds/core.rb
+++ b/lib/cocoaseeds/core.rb
@@ -200,6 +200,10 @@ def github(repo, tag, options={})
           target *self.project.targets.map(&:name) do
             send(__callee__, repo, tag, options)
           end
+        elsif repo.split('/').count != 2
+          raise Seeds::Exception.new\
+          "#{repo}: GitHub should have both username and repo name.\n"\
+          "    (e.g. `devxoul/JLToast`)"
         else
           seed = Seeds::Seed::GitHub.new
           seed.url = "https://github.com/#{repo}"
diff --git a/test/test_core.rb b/test/test_core.rb
index b5d06a7..97df625 100644
--- a/test/test_core.rb
+++ b/test/test_core.rb
@@ -61,6 +61,13 @@ def test_raise_invalid_target
     assert_raises Seeds::Exception do @seed.install end
   end
 
+  def test_raise_invalid_github_reponame
+    seedfile %{
+      github "JLToast", "1.2.2"
+    }
+    assert_raises Seeds::Exception do @seed.install end
+  end
+
   def test_install
     seedfile %{
       github "devxoul/JLToast", "1.2.2", :files => "JLToast/*.{h,swift}"