1 files changed, 10 insertions, 3 deletions
diff --git a/actionpack/lib/action_dispatch/http/mime_negotiation.rb b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
index 0b2b60d2e4..53a98c5d0a 100644
--- a/actionpack/lib/action_dispatch/http/mime_negotiation.rb
+++ b/actionpack/lib/action_dispatch/http/mime_negotiation.rb
@@ -54,8 +54,14 @@ module ActionDispatch
       end
 
       def formats
-        @env["action_dispatch.request.formats"] ||=
-          if parameters[:format]
+        @env["action_dispatch.request.formats"] ||= begin
+          params_readable = begin
+                              parameters[:format]
+                            rescue ActionController::BadRequest
+                              false
+                            end
+
+          if params_readable
             Array(Mime[parameters[:format]])
           elsif use_accept_header && valid_accept_header
             accepts
@@ -64,13 +70,14 @@ module ActionDispatch
           else
             [Mime::HTML]
           end
+        end
       end
 
       # Sets the \variant for template.
       def variant=(variant)
         if variant.is_a?(Symbol)
           @variant = [variant]
-        elsif variant.is_a?(Array) && variant.any? && variant.all?{ |v| v.is_a?(Symbol) }
+        elsif variant.nil? || variant.is_a?(Array) && variant.any? && variant.all?{ |v| v.is_a?(Symbol) }
           @variant = variant
         else
           raise ArgumentError, "request.variant must be set to a Symbol or an Array of Symbols, not a #{variant.class}. " \