Error when using a custom transfer function in neural network toolbox in MATLAB.

Hi,

I defined a custom transfer function (named softplus) which is y = log(1+exp(x)) following this instruction:

http://www.mathworks.com/matlabcentral/answers/91887-how-do-i-add-a-custom-transfer-function-to-the-neural-network-transfer-function-library-in-neural-ne

But when I define the network with this transfer function, I got this error information below:

Undefined function 'exp' for input arguments of type 'char'.

Error in softplus (line 5)
y = log(1+exp(x));

Error in network/subsasgn>getDefaultParam (line 2048)
    param = struct(feval(fcn,'defaultParam'));

Error in network/subsasgn>setLayerTransferFcn (line 1224)
net.layers{i}.transferParam = getDefaultParam(transferFcn);

Error in network/subsasgn>network_subsasgn (line 208)
        if isempty(err), [net,err] = setLayerTransferFcn(net,i,transferFcn); end

Error in network/subsasgn (line 13)
net = network_subsasgn(net,subscripts,v,netname);

Does anyone have idea what is going on here?

Thanks in advance!

[Greg Heath]

It looks like x is not numerical. Type this

 whos x