diff --git a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
index 6be342739f75..2fc388373f12 100644
--- a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
@@ -475,7 +475,9 @@ class TreeChecker extends Phase with SymTransformer {
 }
 
 object TreeChecker {
-  /** Check that TypeParamRefs and MethodParams refer to an enclosing type */
+  /** - Check that TypeParamRefs and MethodParams refer to an enclosing type.
+   *  - Check that all type variables are instantiated.
+   */
   def checkNoOrphans(tp0: Type, tree: untpd.Tree = untpd.EmptyTree)(implicit ctx: Context) = new TypeMap() {
     val definedBinders = new java.util.IdentityHashMap[Type, Any]
     def apply(tp: Type): Type = {
@@ -487,6 +489,7 @@ object TreeChecker {
         case tp: ParamRef =>
           assert(definedBinders.get(tp.binder) != null, s"orphan param: ${tp.show}, hash of binder = ${System.identityHashCode(tp.binder)}, tree = ${tree.show}, type = $tp0")
         case tp: TypeVar =>
+          assert(tp.isInstantiated, s"Uninstantiated type variable: ${tp.show}, tree = ${tree.show}")
           apply(tp.underlying)
         case _ =>
           mapOver(tp)