diff --git a/src/regexp.c b/src/regexp.c
index 5676044..7d0ac60 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -2175,7 +2175,11 @@
 				  if (ret == NULL)
 				      ret = br;
 				  else
+				  {
 				      regtail(lastnode, br);
+				      if (reg_toolong)
+					  return NULL;
+				  }
 
 				  ungetchr();
 				  one_exactly = TRUE;
@@ -2200,6 +2204,8 @@
 				      if (OP(br) == BRANCH)
 				      {
 					  regtail(br, lastbranch);
+					  if (reg_toolong)
+					      return NULL;
 					  br = OPERAND(br);
 				      }
 				      else
diff --git a/src/testdir/test_regexp_utf8.vim b/src/testdir/test_regexp_utf8.vim
index 98b9e73..19ff882 100644
--- a/src/testdir/test_regexp_utf8.vim
+++ b/src/testdir/test_regexp_utf8.vim
@@ -206,3 +206,12 @@
   call assert_equal(1, "\u3042" =~# '[\u3000-\u4000]')
   set re=0
 endfunc
+
+func Test_optmatch_toolong()
+  set re=1
+  " Can only handle about 8000 characters.
+  let pat = '\\%[' .. repeat('x', 9000) .. ']'
+  call assert_fails('call match("abc def", "' .. pat .. '")', 'E339:')
+  set re=0
+endfunc
+
diff --git a/src/version.c b/src/version.c
index 0de9887..7568186 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1720,
+/**/
     1719,
 /**/
     1718,