diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 927a01f..a050467 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -186,15 +186,17 @@
 enddef
 
 def Test_executable()
+  assert_false(executable(""))
+  assert_false(executable(test_null_string()))
+
+  CheckDefExecFailure(['echo executable(123)'], 'E928:')
   CheckDefExecFailure(['echo executable(true)'], 'E928:')
-  CheckDefExecFailure(['echo executable(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo executable("")'], 'E928:')
 enddef
 
 def Test_exepath()
   CheckDefExecFailure(['echo exepath(true)'], 'E928:')
   CheckDefExecFailure(['echo exepath(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo exepath("")'], 'E928:')
+  CheckDefExecFailure(['echo exepath("")'], 'E1142:')
 enddef
 
 def Test_expand()
@@ -254,36 +256,42 @@
 enddef
 
 def Test_filereadable()
+  assert_false(filereadable(""))
+  assert_false(filereadable(test_null_string()))
+
+  CheckDefExecFailure(['echo filereadable(123)'], 'E928:')
   CheckDefExecFailure(['echo filereadable(true)'], 'E928:')
-  CheckDefExecFailure(['echo filereadable(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo filereadable("")'], 'E928:')
 enddef
 
 def Test_filewritable()
+  assert_false(filewritable(""))
+  assert_false(filewritable(test_null_string()))
+
+  CheckDefExecFailure(['echo filewritable(123)'], 'E928:')
   CheckDefExecFailure(['echo filewritable(true)'], 'E928:')
-  CheckDefExecFailure(['echo filewritable(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo filewritable("")'], 'E928:')
 enddef
 
 def Test_finddir()
   CheckDefExecFailure(['echo finddir(true)'], 'E928:')
   CheckDefExecFailure(['echo finddir(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo finddir("")'], 'E928:')
+  CheckDefExecFailure(['echo finddir("")'], 'E1142:')
 enddef
 
 def Test_findfile()
   CheckDefExecFailure(['echo findfile(true)'], 'E928:')
   CheckDefExecFailure(['echo findfile(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo findfile("")'], 'E928:')
+  CheckDefExecFailure(['echo findfile("")'], 'E1142:')
 enddef
 
 def Test_fnamemodify()
+  CheckDefSuccess(['echo fnamemodify(test_null_string(), ":p")'])
+  CheckDefSuccess(['echo fnamemodify("", ":p")'])
+  CheckDefSuccess(['echo fnamemodify("file", test_null_string())'])
+  CheckDefSuccess(['echo fnamemodify("file", "")'])
+
   CheckDefExecFailure(['echo fnamemodify(true, ":p")'], 'E928:')
   CheckDefExecFailure(['echo fnamemodify(v:null, ":p")'], 'E928:')
-  CheckDefExecFailure(['echo fnamemodify("", ":p")'], 'E928:')
   CheckDefExecFailure(['echo fnamemodify("file", true)'], 'E928:')
-  CheckDefExecFailure(['echo fnamemodify("file", v:null)'], 'E928:')
-  CheckDefExecFailure(['echo fnamemodify("file", "")'], 'E928:')
 enddef
 
 def Test_filter_wrong_dict_key_type()
@@ -359,27 +367,35 @@
 enddef
 
 def Test_getfperm()
+  assert_equal('', getfperm(""))
+  assert_equal('', getfperm(test_null_string()))
+
   CheckDefExecFailure(['echo getfperm(true)'], 'E928:')
   CheckDefExecFailure(['echo getfperm(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo getfperm("")'], 'E928:')
 enddef
 
 def Test_getfsize()
+  assert_equal(-1, getfsize(""))
+  assert_equal(-1, getfsize(test_null_string()))
+
   CheckDefExecFailure(['echo getfsize(true)'], 'E928:')
   CheckDefExecFailure(['echo getfsize(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo getfsize("")'], 'E928:')
 enddef
 
 def Test_getftime()
+  assert_equal(-1, getftime(""))
+  assert_equal(-1, getftime(test_null_string()))
+
   CheckDefExecFailure(['echo getftime(true)'], 'E928:')
   CheckDefExecFailure(['echo getftime(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo getftime("")'], 'E928:')
 enddef
 
 def Test_getftype()
+  assert_equal('', getftype(""))
+  assert_equal('', getftype(test_null_string()))
+
   CheckDefExecFailure(['echo getftype(true)'], 'E928:')
   CheckDefExecFailure(['echo getftype(v:null)'], 'E928:')
-  CheckDefExecFailure(['echo getftype("")'], 'E928:')
 enddef
 
 def Test_getqflist_return_type()