First argument must be a vector.; Function call failed.

I am trying to conver a function to C using coder. I am using inbuilt function 'filtfilt'. Function executes properly but when i try to convert it to C i get following errors: 1.Function call failed. 2.First argument must be a vector. 3. Undefined function or variable 'W'. The first assignment to a local variable determines its class. for this line in function: W= filtfilt(digfilt1,1,y1); where digfilt1 and y1 are both of type double.