diff --git a/io/js/src/main/scala/fs2/io/file/Flag.scala b/io/js/src/main/scala/fs2/io/file/Flag.scala
index 027f26ac3d..9d1bd6dc63 100644
--- a/io/js/src/main/scala/fs2/io/file/Flag.scala
+++ b/io/js/src/main/scala/fs2/io/file/Flag.scala
@@ -40,8 +40,8 @@ object Flag extends FlagCompanionApi {
   val Create = Flag(facade.fs.constants.O_CREAT)
   val CreateNew = Flag(facade.fs.constants.O_CREAT.toLong | facade.fs.constants.O_EXCL.toLong)
 
-  val Sync = Flag(facade.fs.constants.O_SYNC)
-  val Dsync = Flag(facade.fs.constants.O_DSYNC)
+  val Sync = Flag(facade.fs.constants.O_SYNC.getOrElse(0.0))
+  val Dsync = Flag(facade.fs.constants.O_DSYNC.getOrElse(0.0))
 
   private[file] implicit val monoid: Monoid[Flag] = new Monoid[Flag] {
     override def combine(x: Flag, y: Flag): Flag = Flag(x.bits | y.bits)
diff --git a/io/js/src/main/scala/fs2/io/internal/facade/fs.scala b/io/js/src/main/scala/fs2/io/internal/facade/fs.scala
index 1c6fa85a0f..3cc25c34a4 100644
--- a/io/js/src/main/scala/fs2/io/internal/facade/fs.scala
+++ b/io/js/src/main/scala/fs2/io/internal/facade/fs.scala
@@ -118,9 +118,11 @@ package fs {
 
     val O_EXCL: Double = js.native
 
-    val O_SYNC: Double = js.native
+    // UndefOr, because windows (:
 
-    val O_DSYNC: Double = js.native
+    val O_SYNC: js.UndefOr[Double] = js.native
+
+    val O_DSYNC: js.UndefOr[Double] = js.native
 
   }