title: invalid model formula in ExtractVars with gam s() wrapper
body: <p>What I am trying to do is simple. Trying to fit a gam model where I can decide the df of the smoomthing spline (the 's' function in the formula part). Calling the gam model should return the fit, but yet it gives error. What am I doing wrong here?</p>
  
  <pre><code>    library(mgcv)
      library(gam)
      set.seed(2) ## simulate some data... 
      dat &lt;- gamSim(1,n=400,dist="normal",scale=2)
      b &lt;- gam(y~s(x0,df = 5)+s(x1)+s(x2)+s(x3),data=dat)
  </code></pre>