diff --git a/compiler/src/dotty/tools/dotc/reporting/Message.scala b/compiler/src/dotty/tools/dotc/reporting/Message.scala
index bed1d955c1a4..43eedaafdc87 100644
--- a/compiler/src/dotty/tools/dotc/reporting/Message.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/Message.scala
@@ -1,7 +1,7 @@
 package dotty.tools
 package dotc
 package reporting
-
+import scala.annotation.threadUnsafe
 import util.SourcePosition
 
 object Message {
@@ -85,10 +85,10 @@ abstract class Message(val errorId: ErrorMessageID) { self =>
   def rawMessage = message
 
   /** The message to report. <nonsensical> tags are filtered out */
-  lazy val message: String = dropNonSensical(msg + msgSuffix)
+  @threadUnsafe lazy val message: String = dropNonSensical(msg + msgSuffix)
 
   /** The explanation to report. <nonsensical> tags are filtered out */
-  lazy val explanation: String = dropNonSensical(explain)
+  @threadUnsafe lazy val explanation: String = dropNonSensical(explain)
 
   /** A message is non-sensical if it contains references to <nonsensical>
    *  tags.  Such tags are inserted by the error diagnostic framework if a
diff --git a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
index 8ce74a6061cc..2d6311f76ebe 100644
--- a/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
+++ b/compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala
@@ -117,12 +117,15 @@ trait TypeAssigner {
 
   /** The type of the selection `tree`, where `qual1` is the typed qualifier part. */
   def selectionType(tree: untpd.RefTree, qual1: Tree)(using Context): Type =
-    var qualType = qual1.tpe.widenIfUnstable
-    if !qualType.hasSimpleKind && tree.name != nme.CONSTRUCTOR then
-      // constructors are selected on typeconstructor, type arguments are passed afterwards
-      qualType = errorType(em"$qualType takes type parameters", qual1.srcPos)
-    else if !qualType.isInstanceOf[TermType] then
-      qualType = errorType(em"$qualType is illegal as a selection prefix", qual1.srcPos)
+    val qualType0 = qual1.tpe.widenIfUnstable
+    val qualType =
+      if !qualType0.hasSimpleKind && tree.name != nme.CONSTRUCTOR then
+        // constructors are selected on typeconstructor, type arguments are passed afterwards
+        errorType(em"$qualType0 takes type parameters", qual1.srcPos)
+      else if !qualType0.isInstanceOf[TermType] && !qualType0.isError then
+        errorType(em"$qualType0 is illegal as a selection prefix", qual1.srcPos)
+      else
+        qualType0
 
     def arrayElemType = qual1.tpe.widen match
       case JavaArrayType(elemtp) => elemtp
diff --git a/tests/neg/i12220.scala b/tests/neg/i12220.scala
new file mode 100644
index 000000000000..c1380b8781b5
--- /dev/null
+++ b/tests/neg/i12220.scala
@@ -0,0 +1,3 @@
+val a: List[Any] = List(List(1,2), List(3,4))
+val _ = for(b <- a ; c <- b.asInstanceOf[List]) { println(c) } // error
+