diff --git a/compiler/typecheck/FamInst.hs b/compiler/typecheck/FamInst.hs
index 49fc5fe8b0eb1f4224445826b65e10882b9e973c..978e92e34c209a5e493d6f0ff0a49264a403c268 100644
--- a/compiler/typecheck/FamInst.hs
+++ b/compiler/typecheck/FamInst.hs
@@ -423,7 +423,8 @@ makeInjectivityErrors
    -> [CoAxBranch] -- ^ List of injectivity conflicts
    -> [(SDoc, SrcSpan)]
 makeInjectivityErrors tycon axiom inj conflicts
-  = let lhs             = coAxBranchLHS axiom
+  = ASSERT2( any id inj, text "No injective type variables" )
+    let lhs             = coAxBranchLHS axiom
         rhs             = coAxBranchRHS axiom
 
         are_conflicts   = not $ null conflicts