How to let Matlab know 'NaN' should be treated as NaN?

I have imported the following .xlsx file into Matlab:

My code:

U_sum24t = zeros(height(Z_24TimeSteps),1);
col_names24tU = Z_24TimeSteps.Properties.VariableNames; 
for k = 1:height(Z_24TimeSteps)
    col_to_sum24tU = any(cell2mat(...
        cellfun(@(x) strcmp(col_names24tU,x),Z_24TimeSteps.U{k},...
        'UniformOutput', false).'),1);
    U_sum24t(k) = sum(Z_24TimeSteps{k,col_to_sum24tU});
end

Following error message occurs:

Undefined function 'sum' for input arguments of type 'cell'. I want to make Matlab treat all 'NaN' as NaN, so I can execute calculations. What I tried already: Replace NaN in Excel with blanks, then in Matlab > import data > "replace blanks with NaN" > not working and still displaying string format