Sass Mixin Error for IE specific filters like -ms-filter

I'm trying to make a button mixin like this:

=default_button(!lighter, !darker) 
  :border= 1px !lighter solid
  :background-color #e3e3e3
  :background= -webkit-gradient(linear, 0 0, 0 100%, from(!lighter), to(!darker)) repeat-x, #d0581e
  :background= -moz-linear-gradient(90deg, !darker, !lighter) repeat-x scroll 0 0 #d0581e
  :filter= progid:DXImageTransform.Microsoft.gradient(startColorstr='!lighter', endColorstr='!darker')
  :-ms-filter= "progid:DXImageTransform.Microsoft.gradient(startColorstr='!lighter', endColorstr='!darker')"
  :zoom 1
  :margin 0 0 0 0
  :width auto
  :padding 2px 14px 2px 14px
  :border-radius 10px
  :-webkit-border-radius 10px
  :-moz-border-radius 10px
  :color #FFF

When I compile the sass, i get this error for the lines beginning with -filter and -ms-filter:

SASS::SyntaxError: Expected rparen token, was single_eq token

I'm pretty sure it's my placement of the ='s, but I'm not exactly sure how to write it correctly. It works if I pass the hex values instead of !lighter, !darker, because then I can remove the = sign like so:

:filter progid:DXImageTransform.Microsoft.gradient(startColorstr='#F89F16', endColorstr='#d0581e')
:-ms-filter "progid:DXImageTransform.Microsoft.gradient(startColorstr='#F89F16', endColorstr='#d0581e')"