diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 2e0b227bba7c..c4efb291da00 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -2261,7 +2261,7 @@ object Types extends TypeUtils {
     def _1: Type
     def _2: Designator
 
-    assert(NamedType.validPrefix(prefix), s"invalid prefix $prefix")
+    if !NamedType.validPrefix(prefix) then throw InvalidPrefix()
 
     private var myName: Name | Null = null
     private var lastDenotation: Denotation | Null = null
@@ -2683,7 +2683,7 @@ object Types extends TypeUtils {
             if (tparams.head.eq(tparam))
               return args.head match {
                 case _: TypeBounds if !widenAbstract => TypeRef(pre, tparam)
-                case arg => arg.boxedUnlessFun(tycon)
+                case arg => arg
               }
             tparams = tparams.tail
             args = args.tail
@@ -3038,6 +3038,8 @@ object Types extends TypeUtils {
       apply(prefix, designatorFor(prefix, name, denot)).withDenot(denot)
   }
 
+  class InvalidPrefix extends Exception
+
   // --- Other SingletonTypes: ThisType/SuperType/ConstantType ---------------------------
 
   /** The type cls.this
diff --git a/compiler/src/dotty/tools/dotc/core/Uniques.scala b/compiler/src/dotty/tools/dotc/core/Uniques.scala
index da6b0aba88bd..b50905c22c98 100644
--- a/compiler/src/dotty/tools/dotc/core/Uniques.scala
+++ b/compiler/src/dotty/tools/dotc/core/Uniques.scala
@@ -3,6 +3,7 @@ package core
 
 import Types.*, Contexts.*, util.Stats.*, Hashable.*, Names.*
 import config.Config
+import Symbols.Symbol
 import Decorators.*
 import util.{WeakHashSet, Stats}
 import WeakHashSet.Entry
@@ -41,8 +42,10 @@ object Uniques:
       val h = doHash(null, designator, prefix)
       if monitored then recordCaching(h, classOf[NamedType])
       def newType =
-        if (isTerm) new CachedTermRef(prefix, designator, h)
-        else new CachedTypeRef(prefix, designator, h)
+        try
+          if isTerm then new CachedTermRef(prefix, designator, h)
+          else new CachedTypeRef(prefix, designator, h)
+        catch case ex: InvalidPrefix => badPrefix(prefix, designator)
       if h == NotCached then newType
       else
         // Inlined from WeakHashSet#put
@@ -61,6 +64,14 @@ object Uniques:
 
         linkedListLoop(oldHead)
       end if
+    end enterIfNew
+
+    private def badPrefix(prefix: Type, desig: Designator)(using Context): Nothing =
+      def name = desig match
+        case desig: Name => desig
+        case desig: Symbol => desig.name
+      throw TypeError(em"invalid prefix $prefix when trying to form $prefix . $name")
+
   end NamedTypeUniques
 
   final class AppliedUniques extends WeakHashSet[AppliedType](Config.initialUniquesCapacity * 2) with Hashable:
diff --git a/tests/neg/i23504.scala b/tests/neg/i23504.scala
new file mode 100644
index 000000000000..e53337eaa105
--- /dev/null
+++ b/tests/neg/i23504.scala
@@ -0,0 +1,3 @@
+def test =
+  Seq.empty[[T] =>> () => ?].head() // error
+  Seq.empty[[T] =>> Int => Int].head(1) // error
\ No newline at end of file