diff --git a/compiler/ccgtypes.nim b/compiler/ccgtypes.nim
index 3d4619764..fa58b5651 100644
--- a/compiler/ccgtypes.nim
+++ b/compiler/ccgtypes.nim
@@ -453,7 +453,7 @@ proc genProcParams(m: BModule, t: PType, rettype, params: var Rope,
       params.add(~"NIM_NOALIAS ")
     params.add(param.loc.r)
     # declare the len field for open arrays:
-    var arr = param.typ
+    var arr = param.typ.skipTypes({tyGenericInst})
     if arr.kind in {tyVar, tyLent, tySink}: arr = arr.lastSon
     var j = 0
     while arr.kind in {tyOpenArray, tyVarargs}:
diff --git a/tests/openarray/topenarray.nim b/tests/openarray/topenarray.nim
index 0b6e3afc4..aec5a4cbf 100644
--- a/tests/openarray/topenarray.nim
+++ b/tests/openarray/topenarray.nim
@@ -23,5 +23,31 @@ proc main =
   when defined(js): discard # xxx bug #15952: `a` left unchanged
   else: doAssert a == [1, 0, 10, 20, 5]
 
+  block: # bug #12521
+    block:
+      type slice[T] = openArray[T]
+
+      # Proc using that alias
+      proc testing(sl: slice[int]): seq[int] =
+        for item in sl:
+          result.add item
+
+      let mySeq = @[1, 2, 3, 4, 5, 6, 7, 8, 9]
+      doAssert testing(mySeq) == mySeq
+      doAssert testing(mySeq[2..^2]) == mySeq[2..^2]
+
+    block:
+      type slice = openArray[int]
+
+      # Proc using that alias
+      proc testing(sl: slice): seq[int] =
+        for item in sl:
+          result.add item
+
+      let mySeq = @[1, 2, 3, 4, 5, 6, 7, 8, 9]
+      doAssert testing(mySeq) == mySeq
+      doAssert testing(mySeq[2..^2]) == mySeq[2..^2]
+
+
 main()
 # static: main() # xxx bug #15952: Error: cannot generate code for: mSlice