I can't explain the weird error message, but the problem seems to be that you've left out the ampersand that should separate the nodes. That is: your $\xymatrix{R\ar@{-->}[r] B}$ should be $\xymatrix{R\ar@{-->}[r] &B}$ In case it matters, I loaded xy-pic with \usepackage[all,cmtip]{xy} Phil Hirschhorn