Why do I get "illegal colormap error"

I am trying to build a movie and I am using im2frame. I do not understand why it is telling me that my colormap is invalid. My data is double precision and I am trying to use colormap gray below is my simple code and error that I am getting. Please help!

for i=1:f
    F(i)=im2frame(mov(:,:,i),gray);
end
Error using im2frame
double-precision indexed CData values must be legal colormap indices: 1.0 <= value
<= length(colormap)