diff --git a/compiler/ccgtypes.nim b/compiler/ccgtypes.nim
index a7b29b9c87..d5745c6983 100644
--- a/compiler/ccgtypes.nim
+++ b/compiler/ccgtypes.nim
@@ -158,8 +158,8 @@ proc getTypeName(m: BModule; typ: PType; sig: SigHash): Rope =
     else:
       break
   let typ = if typ.kind in {tyAlias, tySink, tyOwned}: typ.elementType else: typ
-  ensureMutable typ
   if typ.loc.snippet == "":
+    ensureMutable typ
     typ.typeName(typ.locImpl.snippet)
     typ.locImpl.snippet.add $sig
   else: