diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 0709d9b59238..3c77ef1e9066 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -125,7 +125,9 @@ object Types {
     def isRef(sym: Symbol)(implicit ctx: Context): Boolean = stripAnnots.stripTypeVar match {
       case this1: TypeRef =>
         this1.info match { // see comment in Namer#typeDefSig
-          case TypeAlias(tp) => tp.isRef(sym)
+          case TypeAlias(tp) =>
+            assert((tp ne this) && (tp ne this1), s"$tp / $this")
+            tp.isRef(sym)
           case _ =>  this1.symbol eq sym
         }
       case this1: RefinedOrRecType => this1.parent.isRef(sym)
diff --git a/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala b/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala
index c19bc0556f8a..6841e90b2602 100644
--- a/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala
+++ b/compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala
@@ -44,8 +44,13 @@ class InteractiveDriver(settings: List[String]) extends Driver {
 
   def currentCtx: Context = myCtx
 
-  private val myOpenedFiles = new mutable.LinkedHashMap[URI, SourceFile]
-  private val myOpenedTrees = new mutable.LinkedHashMap[URI, List[SourceTree]]
+  private val myOpenedFiles = new mutable.LinkedHashMap[URI, SourceFile] {
+    override def default(key: URI) = NoSource
+  }
+
+  private val myOpenedTrees = new mutable.LinkedHashMap[URI, List[SourceTree]] {
+    override def default(key: URI) = Nil
+  }
 
   def openedFiles: Map[URI, SourceFile] = myOpenedFiles
   def openedTrees: Map[URI, List[SourceTree]] = myOpenedTrees
diff --git a/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala b/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala
index dd89f9f9f3a1..2863c4e35d0b 100644
--- a/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala
+++ b/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala
@@ -351,9 +351,12 @@ object DottyLanguageServer {
 
   /** Convert an lsp4j.Position to a SourcePosition */
   def sourcePosition(driver: InteractiveDriver, uri: URI, pos: lsp4j.Position): SourcePosition = {
-    val source = driver.openedFiles(uri) // might throw exception
-    val p = Positions.Position(source.lineToOffset(pos.getLine) + pos.getCharacter)
-    new SourcePosition(source, p)
+    val source = driver.openedFiles(uri)
+    if (source.exists) {
+      val p = Positions.Position(source.lineToOffset(pos.getLine) + pos.getCharacter)
+      new SourcePosition(source, p)
+    }
+    else NoSourcePosition
   }
 
   /** Convert a SourcePosition to an lsp4j.Range */