diff --git a/copilot-c99/src/Copilot/Compile/C99/CodeGen.hs b/copilot-c99/src/Copilot/Compile/C99/CodeGen.hs
index 9b9ea8db..7b49cf59 100644
--- a/copilot-c99/src/Copilot/Compile/C99/CodeGen.hs
+++ b/copilot-c99/src/Copilot/Compile/C99/CodeGen.hs
@@ -31,7 +31,7 @@ genfun name expr ty = C.FunDef cty name [] cvars [C.Return $ Just cexpr] where
 mkextdecln :: External -> C.Decln
 mkextdecln (External name _ ty) = decln where
   decln = C.VarDecln (Just C.Extern) cty name Nothing
-  cty   = transtype ty
+  cty   = C.Volatile (transtype ty)
 
 -- | Make a declaration for a copy of an external variable.
 mkextcpydecln :: External -> C.Decln