diff --git a/scalalib/worker/src/mill/scalalib/worker/MockedLookup.scala b/scalalib/worker/src/mill/scalalib/worker/MockedLookup.scala
index 6990a6738e6..fae0efabfe7 100644
--- a/scalalib/worker/src/mill/scalalib/worker/MockedLookup.scala
+++ b/scalalib/worker/src/mill/scalalib/worker/MockedLookup.scala
@@ -13,6 +13,9 @@ case class MockedLookup(am: VirtualFile => Optional[CompileAnalysis])
   override def analysis(classpathEntry: VirtualFile): Optional[CompileAnalysis] =
     am(classpathEntry)
 
-  override def definesClass(classpathEntry: VirtualFile): DefinesClass =
-    Locate.definesClass(classpathEntry)
+  override def definesClass(classpathEntry: VirtualFile): DefinesClass = {
+    if (classpathEntry.name.toString != "rt.jar")
+      Locate.definesClass(classpathEntry)
+    else false
+  }
 }
diff --git a/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala b/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
index 4aff9d9a04a..91521bd0cda 100644
--- a/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
+++ b/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
@@ -474,7 +474,7 @@ class ZincWorkerImpl(
 
     val store = FileAnalysisStore.binary(zincFile.toIO)
 
-    val converter = PlainVirtualFileConverter.converter
+    val converter = MappedFileConverter.empty
     val classpath = (compileClasspath.iterator ++ Some(classesDir))
       .map(path => converter.toVirtualFile(path.toNIO))
       .toArray