diff --git a/test/api_key_test.rb b/test/api_key_test.rb
index fea3fd0..1daa13e 100644
--- a/test/api_key_test.rb
+++ b/test/api_key_test.rb
@@ -40,6 +40,12 @@
         Google::Auth::APIKeyCredentials.new
       end.must_raise ArgumentError
     end
+
+    it "raises if API key is empty" do
+      expect do
+        Google::Auth::APIKeyCredentials.new(api_key: "")
+      end.must_raise ArgumentError
+    end
   end
 
   describe "#from_env" do