Error message for extrapolation?

I keep getting an error message when using the interp1 function. For some background, I have 107 files that all have the same variables but different lengths. So I'm trying to extrapolate the variable in each file to have the same lengths.

The error message I keep getting:

Error using interp1>sanitycheckmethod (line 253)
Invalid interpolation method.

Error in interp1>parseinputs (line 376)
        method = sanitycheckmethod(varargin{end});

Error in interp1 (line 78)
[method,extrapval,ndataarg,pp] = parseinputs(varargin{:});

Error in PhysOceanFinal (line 54)
       zonal1.depths=interp1(zonal1.density,zonal1.depth,[zonal1.depth(end) depths(end),'linear','extrap']);

My code:

direc1=dir('*.mat');
%calculate northward geostrophic velocity using thermal wind relation
%calculate drho/dz by integrating the density 
%First interpolate density into uniform depth values
%Use largest depth file as primary depth 
load('zonal1_240.mat'); %use this files depth values
depths=zonal1.depth;
%use extrapolate function to get density profile
for i=1:length(direc1)
    load(direc1(i).name);
    if length(zonal1.depth)<length(depths)
       zonal1.depths=interp1(zonal1.depth,zonal1.density,[zonal1.depth(end) depths(end),'linear','extrap']);
    end 
end 

Answer 1

[Walter Roberson]

       zonal1.depths=interp1(zonal1.depth,zonal1.density,[zonal1.depth(end) depths(end),'linear','extrap']);

The] is in the wrong place, needs to be before ,'linear'