为什么map为空

当我用[I,map]=imread('1.tif');读入文件1.tif之后，
发现map为空，用imshow(I)可以显示图形，但是用I1=ind2gray(I,map)时报错：
??? Error using ==> images\private\checkinput>check_attributes
Function IND2GRAY expected its second input argument, MAP,
to be nonempty.

Error in ==> images\private\checkinput at 37
check_attributes(A, attributes, function_name, variable_name, ...

Error in ==> ind2gray>parse_inputs at 80
checkinput(varargin{2},{'double','uint8','uint16'},{'nonempty','2d'}, ...

Error in ==> ind2gray at 23
[a,cm] = parse_inputs(varargin{:});

不知道这是为什么，希望大家指导一下哈