diff --git a/src/evalfunc.c b/src/evalfunc.c
index d78589c..f310520 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -494,6 +494,7 @@
     if (type->tt_type == VAR_FUNC
 	    && !(type->tt_member->tt_type == VAR_BOOL
 		|| type->tt_member->tt_type == VAR_NUMBER
+		|| type->tt_member->tt_type == VAR_UNKNOWN
 		|| type->tt_member->tt_type == VAR_ANY))
     {
 	arg_type_mismatch(&t_func_bool, type, context->arg_idx + 1);
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 6cad1e4..bc59e3a 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -1253,6 +1253,12 @@
   assert_equal([], filter([1, 2, 3], '0'))
   assert_equal([1, 2, 3], filter([1, 2, 3], '1'))
   assert_equal({b: 20}, filter({a: 10, b: 20}, 'v:val == 20'))
+
+  def GetFiltered(): list<number>
+    var Odd: func = (_, v) => v % 2
+    return range(3)->filter(Odd)
+  enddef
+  assert_equal([1], GetFiltered())
 enddef
 
 def Test_filter_wrong_dict_key_type()
diff --git a/src/version.c b/src/version.c
index 8c4377b..0d99bb8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3911,
+/**/
     3910,
 /**/
     3909,