Invalid XHTML markup for readonly attribute

http://validator.w3.org/ on user/register page says:

• # Error Line 105, Column 81: syntax of attribute value does not conform to declared value.
  …ons" id="edit-conditions" readonly="" class="form-textarea resizable">Dadada
  The value of an attribute contained something that is not allowed by the specified syntax for that type of attribute. For instance, the “selected” attribute must be either minimized as “selected” or spelled out in full as “selected="selected"”; the variant “selected=""” is not allowed.
• # Error Line 105, Column 81: value of attribute "readonly" cannot be ""; must be one of "readonly".
  …ons" id="edit-conditions" readonly="" class="form-textarea resizable">Dadada