diff --git a/lib/slack/web/api/endpoints/dialog.rb b/lib/slack/web/api/endpoints/dialog.rb
index 01f9dfd..d017adf 100644
--- a/lib/slack/web/api/endpoints/dialog.rb
+++ b/lib/slack/web/api/endpoints/dialog.rb
@@ -17,6 +17,12 @@ module Slack
           def dialog_open(options = {})
             throw ArgumentError.new('Required arguments :dialog missing') if options[:dialog].nil?
             throw ArgumentError.new('Required arguments :trigger_id missing') if options[:trigger_id].nil?
+            # dialog must be passed as an encoded JSON string
+            if options.key?(:dialog)
+              dialog = options[:dialog]
+              dialog = JSON.dump(dialog) unless dialog.is_a?(String)
+              options = options.merge(dialog: dialog)
+            end
             post('dialog.open', options)
           end
         end