Catches nil api_key values during Client initialization

lib/recurly/client.rb

@@ -55,6 +55,8 @@ class Client
     # @param api_key [String] The private API key
     # @param logger [Logger] A logger to use. Defaults to creating a new STDOUT logger with level WARN.
     def initialize(api_key:, logger: nil)
+      raise ArgumentError, "'api_key' must be set to a non-nil value" if api_key.nil?
+
       set_api_key(api_key)
 
       if logger.nil?

spec/recurly/client_spec.rb

@@ -388,6 +388,18 @@
     end
   end
 
+  context "with a missing api key" do
+    describe "initialize" do
+      let(:options) { { api_key: nil } }
+
+      it "should raise an ArgumentError when api_key is nil" do
+        expect {
+          Recurly::Client.new(**options)
+        }.to raise_error(ArgumentError)
+      end
+    end
+  end
+
   describe "unexpected HTML responses" do
     context "with 200 OK" do
       let(:response) do