diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index 5df373a01e4a..612a5ce86e21 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -210,6 +210,8 @@ class Definitions {
     cls
   }
   @tu lazy val ScalaPackageObject: Symbol = requiredModule("scala.package")
+  @tu lazy val ScalaRuntimePackageVal: TermSymbol = requiredPackage("scala.runtime")
+  @tu lazy val ScalaRuntimePackageClass: ClassSymbol = ScalaRuntimePackageVal.moduleClass.asClass
   @tu lazy val JavaPackageVal: TermSymbol = requiredPackage(nme.java)
   @tu lazy val JavaPackageClass: ClassSymbol = JavaPackageVal.moduleClass.asClass
   @tu lazy val JavaLangPackageVal: TermSymbol = requiredPackage(jnme.JavaLang)
@@ -1098,6 +1100,9 @@ class Definitions {
   def isTupleClass(cls: Symbol): Boolean = isVarArityClass(cls, str.Tuple)
   def isProductClass(cls: Symbol): Boolean = isVarArityClass(cls, str.Product)
 
+  def isBoxedUnitClass(cls: Symbol): Boolean =
+    cls.isClass && (cls.owner eq ScalaRuntimePackageClass) && cls.name == tpnme.BoxedUnit
+
   def isScalaShadowingPackageClass(cls: Symbol): Boolean =
     cls.name == tpnme.scalaShadowing && cls.owner == RootClass
 
@@ -1314,6 +1319,11 @@ class Definitions {
     else parents
   }
 
+  /** If it is BoxedUnit, remove `java.io.Serializable` from `parents`. */
+  def adjustForBoxedUnit(cls: ClassSymbol, parents: List[Type]): List[Type] =
+    if (isBoxedUnitClass(cls)) parents.filter(_.typeSymbol != JavaSerializableClass)
+    else parents
+
   private val HasProblematicGetClass: Set[Name] = Set(
     tpnme.AnyVal, tpnme.Byte, tpnme.Short, tpnme.Char, tpnme.Int, tpnme.Long, tpnme.Float, tpnme.Double,
     tpnme.Unit, tpnme.Boolean)
diff --git a/compiler/src/dotty/tools/dotc/core/StdNames.scala b/compiler/src/dotty/tools/dotc/core/StdNames.scala
index 87a1b6e25161..04b2bfdc74fb 100644
--- a/compiler/src/dotty/tools/dotc/core/StdNames.scala
+++ b/compiler/src/dotty/tools/dotc/core/StdNames.scala
@@ -306,6 +306,7 @@ object StdNames {
     final val Nil: N                = "Nil"
     final val Predef: N             = "Predef"
     final val ScalaRunTime: N       = "ScalaRunTime"
+    final val BoxedUnit: N          = "BoxedUnit"
     final val Some: N               = "Some"
 
     val x_0 : N  = "x$0"
diff --git a/compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala b/compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
index 0cb50e868d01..17f6731ffe49 100644
--- a/compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
+++ b/compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala
@@ -98,10 +98,11 @@ object Scala2Unpickler {
     val tempInfo = new TempClassInfo(denot.owner.thisType, cls, decls, ost)
     denot.info = tempInfo // first rough info to avoid CyclicReferences
     val parents1 = if (parents.isEmpty) defn.ObjectType :: Nil else parents.map(_.dealias)
-    // Add extra parents to the tuple classes from the standard library
+    // Adjust parents of the tuple classes and BoxedUnit from the standard library
+    // If from Scala 2, adjust for tuple classes; if not, it's from Java, and adjust for BoxedUnit
     val normalizedParents =
       if (fromScala2) defn.adjustForTuple(cls, tparams, parents1)
-      else parents1 // We are setting the info of a Java class, so it cannot be one of the tuple classes
+      else defn.adjustForBoxedUnit(cls, parents1)
     for (tparam <- tparams) {
       val tsym = decls.lookup(tparam.name)
       if (tsym.exists) tsym.setFlag(TypeParam)
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index c1afdb22b08d..f488c417d049 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -1170,7 +1170,10 @@ class Namer { typer: Typer =>
       denot.info = tempInfo
 
       val parentTypes = defn.adjustForTuple(cls, cls.typeParams,
-        ensureFirstIsClass(parents.map(checkedParentType(_)), cls.span))
+        defn.adjustForBoxedUnit(cls,
+          ensureFirstIsClass(parents.map(checkedParentType(_)), cls.span)
+        )
+      )
       typr.println(i"completing $denot, parents = $parents%, %, parentTypes = $parentTypes%, %")
 
       if (impl.derived.nonEmpty) {
diff --git a/project/Build.scala b/project/Build.scala
index 3003c44fff74..dd5b55871190 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -1054,7 +1054,7 @@ object Build {
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/utils" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/junit" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/niobuffer" ** "*.scala").get
-          ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/niocharset" ** (("*.scala": FileFilter)  -- "BaseCharsetTest.scala" -- "Latin1Test.scala" -- "USASCIITest.scala" -- "UTF16Test.scala" -- "UTF8Test.scala")).get
+          ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/niocharset" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/scalalib" ** (("*.scala": FileFilter) -- "EnumerationTest.scala" -- "SymbolTest.scala")).get
           ++ (dir / "shared/src/test/require-sam" ** "*.scala").get
           ++ (dir / "shared/src/test/require-jdk8/org/scalajs/testsuite/compiler" ** "*.scala").get