diff --git a/compiler/GHC/CmmToAsm/X86/CodeGen.hs b/compiler/GHC/CmmToAsm/X86/CodeGen.hs
index 3d65da373c72307e5176b71c3626e3a68cf29a15..cc7e255e80a99c10626fd2029634edf6b6e50baf 100644
--- a/compiler/GHC/CmmToAsm/X86/CodeGen.hs
+++ b/compiler/GHC/CmmToAsm/X86/CodeGen.hs
@@ -976,7 +976,7 @@ getRegister' _ is32Bit (CmmMachOp mop [x, y]) = -- dyadic MachOps
                            W16 -> 15
                            W32 -> 31
                            W64 -> 63
-                           _ -> panic "shift_amt"
+                           w -> panic ("shift_amt: " ++ show w)
 
              format = intFormat rep
              code = a_code `appOL` b_code eax `appOL`