[racket-dev] variable wrong procedure or structure-type shape error?

Hmmmm…. something changed between Racket 6.1 and the latest release that is producing this mysterious (to me at least!) error in a project of mine that compiles fine with 6.1, but not with the latest version, producing: 

Welcome to DrRacket, version 6.1.0.5--2014-08-28(a1f5340/a) [3m].
Language: racket [custom]; memory limit: 512 MB.
link: bad variable linkage;
 reference to a variable that has the wrong procedure or structure-type shape
  reference phase level: 0
  variable module: "/Users/lysseus/Racket/games/lib/matrix.rkt"
  variable phase: 0
  reference in module: "/Users/lysseus/Racket/games/2048/2048-obj.rkt" in: matrix-row

Any idea wheat this error means and why 6.1.0.5 is getting the error? It appears to have been introduced sometime after version 6.1.0.5--2014-08-25(32ae3f8/a) [3m]., which compiled fine.

-Kevin