After installing the cvx, this problem came up when testing a simple model. And it come up again every time i try a problem.Here is the error in testing simple model:

```
Testing with a simple model...
UNEXPECTED ERROR: ---------------------------------------------
Undefined function 'vec' for input arguments of type 'double'.
Error in cvx_sdpt3>solve (line 405)
x(1,tvec{k}) = x(1,tvec{k}) + vec(xx{k})' * xvec{k};
Error in cvxprob/solve (line 429)
[ x, status, tprec, iters ] = shim.solve( At, b, c,
cones, quiet, prec, solv.settings, eargs{:} );
Error in cvx_end (line 88)
solve( prob );
Error in cvx_setup (line 213)
cvx_end
```