diff --git a/compiler/src/dotty/tools/dotc/typer/Checking.scala b/compiler/src/dotty/tools/dotc/typer/Checking.scala
index 700bd483ff38..761e7cdab37c 100644
--- a/compiler/src/dotty/tools/dotc/typer/Checking.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Checking.scala
@@ -611,6 +611,8 @@ object Checking {
     val mods = mdef.mods
     def flagSourcePos(flag: FlagSet) =
       mods.mods.find(_.flags == flag).getOrElse(mdef).srcPos
+    if mods.is(Open) then
+      report.error(ModifierNotAllowedForDefinition(Open), flagSourcePos(Open))
     if mods.is(Abstract) then
       report.error(ModifierNotAllowedForDefinition(Abstract), flagSourcePos(Abstract))
     if mods.is(Sealed) then
diff --git a/tests/neg/i21760.scala b/tests/neg/i21760.scala
new file mode 100644
index 000000000000..625e03520dfb
--- /dev/null
+++ b/tests/neg/i21760.scala
@@ -0,0 +1 @@
+open object O // error
\ No newline at end of file