[R] Inscrutable error message in mgcv: 1> prediction = predict(MI, se.fit=TRUE, newdata=rhc), Error in if (object$inter) X[[i]] <- PredictMat(object$margin[[i]], dat, : , argument is of length zero

[Andrew Crane-Droesch]

Dear List,

I am trying to reproduce a figure that I made for an analysis that I did 
a few months ago.  Between when I first made the figure and now, I've 
upgraded to R 3.0.2 and upgraded my operating system (ubuntu 13.04).  My 
codebase, which once works, is throwing an error when I try to use 
predict.gam on a model object that I saved:

1> prediction = predict(MI,se.fit=TRUE,newdata=rhc)
Error in if (object$inter) X[[i]] <- PredictMat(object$margin[[i]], dat,  :
   argument is of length zero

traceback() gives me the following:

1> traceback()
6: Predict.matrix.tensor.smooth(object, dk$data)
5: Predict.matrix(object, dk$data)
4: Predict.matrix3(object, data)
3: PredictMat(object$smooth[[k]], data)
2: predict.gam(MI, se.fit = TRUE, newdata = rhc)
1: predict(MI, se.fit = TRUE, newdata = rhc)

the final function being called looks like this:

1> Predict.matrix.tensor.smooth
function (object, data)
{
     m <- length(object$margin)
     X <- list()
     for (i in 1:m) {
         term <- object$margin[[i]]$term
         dat <- list()
         for (j in 1:length(term)) dat[[term[j]]] <- data[[term[j]]]
         if (object$inter)
             X[[i]] <- PredictMat(object$margin[[i]], dat, n = 
length(dat[[1]]))
         else X[[i]] <- Predict.matrix(object$margin[[i]], dat)
     }
     mxp <- length(object$XP)
     if (mxp > 0)
         for (i in 1:mxp) if (!is.null(object$XP[[i]]))
             X[[i]] <- X[[i]] %*% object$XP[[i]]
     T <- tensor.prod.model.matrix(X)
     T
}

Unfortunately, I can't say that I understand how that function is 
working, beyond that it takes a fitted model object and makes a piece of 
the model matrix.

Any ideas about what this could stem from?  Where to start looking to 
fix it?  I could probably do the entire analysis from scratch, but it is 
quite complex and I'd prefer to save the time.

Apologies for non-reproducible code -- the data is big and the script is 
long.

Thanks for any assistance,
Andrew