Stop matlab crashing when function input is not a number

I've tried isnan(), isnumeric() and the code below, yet whenever I enter L as a letter, Matlab crashed and returns and error. Why does the below code not work?

function [x]= functionname(L,C,R,V,F,State)

if isa(L,'numeric')

Do code

else

x='Error'

Answer 1

[Image Analyst]

Perhaps this:

% Ask user for a number.
defaultValue = 45;
titleBar = 'Enter a value';
userPrompt = 'Enter the integer';
caUserInput = inputdlg(userPrompt, titleBar, 1, {num2str(defaultValue)});
if isempty(caUserInput),return,end; % Bail out if they clicked Cancel.
integerValue = round(str2double(cell2mat(caUserInput)));
% Check for a valid integer.
if isnan(integerValue)
    % They didn't enter a number.  
    % They clicked Cancel, or entered a character, symbols, or something else not allowed.
    integerValue = defaultValue;
    message = sprintf('I said it had to be an integer.\nI will use %d and continue.', integerValue);
    uiwait(warndlg(message));
end