diff --git a/lib/killbill_client/api/net_http_adapter.rb b/lib/killbill_client/api/net_http_adapter.rb
index c5359fc..68b23c7 100644
--- a/lib/killbill_client/api/net_http_adapter.rb
+++ b/lib/killbill_client/api/net_http_adapter.rb
@@ -157,7 +157,7 @@ def encode_params(options = {})
             options[:params][:withStackTrace] = true
           end
 
-          pairs = options[:params].filter_map { |key, value|
+          pairs = options[:params].map { |key, value|
             next if value.nil?
 
             # If the value is an array, we 'demultiplex' into several
@@ -169,7 +169,7 @@ def encode_params(options = {})
             else
               "#{CGI.escape key.to_s}=#{CGI.escape value.to_s}"
             end
-          }
+          }.compact
           pairs.flatten!
           return nil if pairs.empty?
           "?#{pairs.join '&'}"
diff --git a/lib/killbill_client/models/invoice.rb b/lib/killbill_client/models/invoice.rb
index d0b0e6f..8832576 100644
--- a/lib/killbill_client/models/invoice.rb
+++ b/lib/killbill_client/models/invoice.rb
@@ -212,7 +212,7 @@ def cancel_subscription_dry_run(account_id, bundle_id, subscription_id, target_d
         def get_invoice_template(is_manual_pay, locale = nil, options = {})
 
           require_multi_tenant_options!(options, "Retrieving an invoice template supported in multi-tenant mode")
-
+          locale ||= 'en'
 
           get "#{KILLBILL_API_INVOICES_PREFIX}/#{is_manual_pay ? "manualPayTemplate/#{locale}" : "template"}",
               {},