diff --git a/src/eval.c b/src/eval.c
index 8b1ceae..8670e12 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1704,7 +1704,10 @@
 						       &stringval, opt_flags);
 		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 */
diff --git a/src/testdir/test_assign.vim b/src/testdir/test_assign.vim
index efc9d08..317d458 100644
--- a/src/testdir/test_assign.vim
+++ b/src/testdir/test_assign.vim
@@ -29,3 +29,17 @@
   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
diff --git a/src/version.c b/src/version.c
index 925a6cd..6e3f90a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1172,
+/**/
     1171,
 /**/
     1170,