diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 49908bacdcd6..fbd74fa94f9c 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -3556,6 +3556,8 @@ object Types {
             else lo.applyIfParameterized(args)
           case _ => NoType
         }
+      case tycon: AppliedType =>
+        tycon.lowerBound.applyIfParameterized(args)
       case _ =>
         NoType
     }
diff --git a/tests/pos/i6451.scala b/tests/pos/i6451.scala
new file mode 100644
index 000000000000..4ab1cc93209e
--- /dev/null
+++ b/tests/pos/i6451.scala
@@ -0,0 +1,10 @@
+object Test {
+
+  type YZ = [Y[_]] => [Z[_]] => [T] => Y[Z[T]]
+
+  val r1: List[List[Int]] = ???
+  def r2(): List[List[Int]] = ???
+
+  val l1: YZ[List][List][Int] = r1
+  val l2: YZ[List][List][Int] = r2()
+}
\ No newline at end of file