diff --git a/actionmailer/lib/action_mailer/old_api.rb b/actionmailer/lib/action_mailer/old_api.rb
index dedb447ced3844b8d0044166a3581bffd69152ad..2a47e4d5a6f2973ce526fb5491cab303c5f41d57 100644
--- a/actionmailer/lib/action_mailer/old_api.rb
+++ b/actionmailer/lib/action_mailer/old_api.rb
@@ -247,7 +247,7 @@ def parse_content_type(defaults=nil)
         [ nil, {} ]
       else
         ctype, *attrs = @content_type.split(/;\s*/)
-        attrs = Hash[attrs.map { |attr| attr.split(/\=/, 2) }]
+        attrs = Hash[attrs.map { |attr| attr.split(/=/, 2) }]
         [ctype, {"charset" => @charset}.merge!(attrs)]
       end
     end