diff --git a/lib/stripe/api_operations/request.rb b/lib/stripe/api_operations/request.rb
index 6abe838f0..579da7c95 100644
--- a/lib/stripe/api_operations/request.rb
+++ b/lib/stripe/api_operations/request.rb
@@ -8,6 +8,8 @@ def request(method, url, params = {}, opts = {})
           warn_on_opts_in_params(params)
 
           opts = Util.normalize_opts(opts)
+          error_on_non_string_user_opts(opts)
+
           opts[:client] ||= StripeClient.active_client
 
           headers = opts.clone
@@ -31,10 +33,24 @@ def request(method, url, params = {}, opts = {})
           [resp, opts_to_persist]
         end
 
+        private def error_on_non_string_user_opts(opts)
+          Util::OPTS_USER_SPECIFIED.each do |opt|
+            next unless opts.key?(opt)
+
+            val = opts[opt]
+            next if val.nil?
+            next if val.is_a?(String)
+
+            raise ArgumentError,
+                  "request option '#{opt}' should be a string value " \
+                    "(was a #{val.class})"
+          end
+        end
+
         private def warn_on_opts_in_params(params)
           Util::OPTS_USER_SPECIFIED.each do |opt|
             if params.key?(opt)
-              warn("WARNING: #{opt} should be in opts instead of params.")
+              warn("WARNING: '#{opt}' should be in opts instead of params.")
             end
           end
         end
diff --git a/test/stripe/api_resource_test.rb b/test/stripe/api_resource_test.rb
index 82b4a3bca..c0dfbc163 100644
--- a/test/stripe/api_resource_test.rb
+++ b/test/stripe/api_resource_test.rb
@@ -227,6 +227,23 @@ class NestedTestAPIResource < APIResource
         end
       end
 
+      should "error if a user-specified opt is given a non-nil non-string value" do
+        stub_request(:post, "#{Stripe.api_base}/v1/charges")
+          .to_return(body: JSON.generate(charge_fixture))
+
+        # Works fine if not included or a string.
+        Stripe::Charge.create({ amount: 100, currency: "usd" }, {})
+        Stripe::Charge.create({ amount: 100, currency: "usd" }, idempotency_key: "12345")
+
+        # Errors on a non-string.
+        e = assert_raises(ArgumentError) do
+          Stripe::Charge.create({ amount: 100, currency: "usd" }, idempotency_key: :foo)
+        end
+        assert_equal "request option 'idempotency_key' should be a string value " \
+                       "(was a Symbol)",
+                     e.message
+      end
+
       should "requesting with a unicode ID should result in a request" do
         stub_request(:get, "#{Stripe.api_base}/v1/customers/%E2%98%83")
           .to_return(body: JSON.generate(make_missing_id_error), status: 404)