diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index b4eeea6..cfae760 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -599,6 +599,11 @@
   call assert_fails("let v = -{}", 'E728:')
 endfunc
 
+func Test_white_in_function_call()
+  let text = substitute ( 'some text' , 't' , 'T' , 'g' )
+  call assert_equal('some TexT', text)
+endfunc
+
 " Test for float value comparison
 func Test_float_compare()
   CheckFeature float
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index aa705b2..746983d 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1686,6 +1686,10 @@
   assert_equal([0, 1, 2], --3->range())
 
   call CheckDefFailure(["let x = 'yes'->Echo"], 'E107:')
+  call CheckScriptFailure([
+	"vim9script",
+	"let x = substitute ('x', 'x', 'x', 'x')"
+	], 'E121:')
 enddef