diff --git a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
index 967b35f4f87b..bfcd8a152fb9 100644
--- a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -406,8 +406,14 @@ trait Inferencing { this: Typer =>
    */
   def interpolateTypeVars(tree: Tree, pt: Type, locked: TypeVars)(implicit ctx: Context): tree.type = {
     val state = ctx.typerState
-    if (state.ownedVars.size > locked.size) {
-      val qualifying = state.ownedVars -- locked
+
+    // Note that some variables in `locked` might not be in `state.ownedVars`
+    // anymore if they've been garbage-collected, so we can't use
+    // `state.ownedVars.size > locked.size` as an early check to avoid computing
+    // `qualifying`.
+    val qualifying = state.ownedVars -- locked
+
+    if (!qualifying.isEmpty) {
       typr.println(i"interpolate $tree: ${tree.tpe.widen} in $state, owned vars = ${state.ownedVars.toList}%, %, previous = ${locked.toList}%, % / ${state.constraint}")
       val resultAlreadyConstrained =
         tree.isInstanceOf[Apply] || tree.tpe.isInstanceOf[MethodOrPoly]
diff --git a/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala b/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala
index a857c2163006..2161d597e150 100644
--- a/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala
+++ b/compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala
@@ -7,6 +7,7 @@ import collection.mutable.ListBuffer
  */
 abstract class SimpleIdentitySet[+Elem <: AnyRef] {
   def size: Int
+  def isEmpty: Boolean = size == 0
   def + [E >: Elem <: AnyRef](x: E): SimpleIdentitySet[E]
   def - [E >: Elem <: AnyRef](x: E): SimpleIdentitySet[Elem]
   def contains[E >: Elem <: AnyRef](x: E): Boolean
diff --git a/tests/pos/i6247.scala b/tests/pos/i6247.scala
new file mode 100644
index 000000000000..20a3ae091692
--- /dev/null
+++ b/tests/pos/i6247.scala
@@ -0,0 +1,10 @@
+class Foo {
+  implicit class NN[T](x:T|Null) {
+    def nn: T = x.asInstanceOf[T]
+  }
+
+  val x1: String|Null = null
+  x1.nn.length
+  val x2: String = x1.nn
+  x1.nn.length
+}