Pattern match failure in StgCmmExpr.hs when adding new out-of-line primop

Hi guys, I'm trying to add a new primop; as a starting point I just copied the definition of "catch#" in primops.txt.pp and renamed "catch#" to "catchRethrow#", and added the corresponding declarations following https://ghc.haskell.org/trac/ghc/wiki/Commentary/PrimOps#AddinganewPrimOp ; however, when I do this, I get ghc-stage1: panic! (the 'impossible' happened) (GHC version 7.7.20131108 for x86_64-apple-darwin): compiler/codeGen/StgCmmExpr.hs:508:15-48: Irrefutable pattern failed for pattern (CoreSyn.DEFAULT, deflt) I can't quite figure out why that is happening and how I should address it. Any suggestions? Thanks! -E