digital differentiator using remez

[Ron Crummett]

Hi -

I am trying to design a 4th order wideband differentiator using the 
remez function.  I type in
   >>b = remez(4, [0 0.75], [0 0.75*pi], [], 'differentiator');

And I get this response:
   error: invalid conversion from matrix to real vector
   error: remez: need one weight for each band [=length(band)/2]
   error: evaluating assignment expression near line 22, column 3

My only guess is that I need something in the weighting entry, so I 
change it to
   >> b = remez(4, [0 0.75], [0 0.75*pi], 1, 'differentiator');

Which gives me the message:
   error: remez: griddensity is too low; must be greater than 16
   error: evaluating assignment expression near line 22, column 3

A third attempt:
   >> b = remez(4, [0 0.75], [0 0.75*pi], 1, 'differentiator', 64);
   error: remez: incorrect argument list
   error: evaluating assignment expression near line 22, column 3

At this point I am out of ideas of what I need to change.  Has anyone 
had any success with this?  I am running 2.9.6 on Kubuntu breezy.  Thanks.

-Ron