diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index 690a3f7..eb30230 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -2459,6 +2459,30 @@
   call RunTest_mapping_works_with_mods(function('GetEscCodeCSIu'), 'C-S-A', 8)
 endfunc
 
+func Test_mapping_works_with_unknown_modifiers()
+  new
+  set timeoutlen=10
+
+  for Func in [function('GetEscCodeCSI27'), function('GetEscCodeCSIu')]
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5)
+    " Add 16, 32, 64 or 128 for modifiers we currently don't support.
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 16)
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 32)
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 64)
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 128)
+
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2)
+    " Add 16, 32, 64 or 128 for modifiers we currently don't support.
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 16)
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 32)
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 64)
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 128)
+  endfor
+
+  bwipe!
+  set timeoutlen&
+endfunc
+
 func Test_insert_literal()
   set timeoutlen=10