diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index b7f5d93bf3..0b81e10bd9 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -1912,6 +1912,7 @@ static char_u *ex_let_one(char_u *arg, typval_T *const tv,
         if ((opt_type == 1 && *op == '.')
             || (opt_type == 0 && *op != '.')) {
           EMSG2(_(e_letwrong), op);
+          s = NULL;  // don't set the value
         } else {
           if (opt_type == 1) {  // number
             if (*op == '+') {
diff --git a/src/nvim/testdir/test_assign.vim b/src/nvim/testdir/test_assign.vim
index a75e89d5dd..50415ad6fd 100644
--- a/src/nvim/testdir/test_assign.vim
+++ b/src/nvim/testdir/test_assign.vim
@@ -31,3 +31,17 @@ func Test_let_termcap()
   let &t_xx = ""
   call assert_fails('let x = &t_xx', 'E15')
 endfunc
+
+func Test_let_option_error()
+  let _w = &tw
+  let &tw = 80
+  call assert_fails('let &tw .= 1', 'E734')
+  call assert_equal(80, &tw)
+  let &tw = _w
+
+  let _w = &fillchars
+  let &fillchars = "vert:|"
+  call assert_fails('let &fillchars += "diff:-"', 'E734')
+  call assert_equal("vert:|", &fillchars)
+  let &fillchars = _w
+endfunc