diff --git a/src/nvim/autocmd.c b/src/nvim/autocmd.c
index 7a13399c6b..e154095001 100644
--- a/src/nvim/autocmd.c
+++ b/src/nvim/autocmd.c
@@ -931,6 +931,10 @@ int do_autocmd_event(event_T event, const char *pat, bool once, int nested, cons
     if (is_buflocal) {
       const int buflocal_nr = aupat_get_buflocal_nr(pat, patlen);
 
+      if (buflocal_nr == 0 || buflist_findnr(buflocal_nr) == NULL) {
+        semsg(_(e_buffer_nr_invalid_buffer_number), buflocal_nr);
+        return FAIL;
+      }
       // normalize pat into standard "<buffer>#N" form
       aupat_normalize_buflocal_pat(buflocal_pat, pat, patlen, buflocal_nr);
 
@@ -1000,6 +1004,10 @@ int autocmd_register(int64_t id, event_T event, const char *pat, int patlen, int
   if (is_buflocal) {
     buflocal_nr = aupat_get_buflocal_nr(pat, patlen);
 
+    if (buflocal_nr == 0 || buflist_findnr(buflocal_nr) == NULL) {
+      semsg(_(e_buffer_nr_invalid_buffer_number), buflocal_nr);
+      return FAIL;
+    }
     // normalize pat into standard "<buffer>#N" form
     aupat_normalize_buflocal_pat(buflocal_pat, pat, patlen, buflocal_nr);
 
@@ -1025,12 +1033,6 @@ int autocmd_register(int64_t id, event_T event, const char *pat, int patlen, int
 
   // No matching pattern found, allocate a new one.
   if (ap == NULL) {
-    // refuse to add buffer-local ap if buffer number is invalid
-    if (is_buflocal && (buflocal_nr == 0 || buflist_findnr(buflocal_nr) == NULL)) {
-      semsg(_("E680: <buffer=%d>: invalid buffer number "), buflocal_nr);
-      return FAIL;
-    }
-
     ap = xmalloc(sizeof(AutoPat));
 
     if (is_buflocal) {
@@ -2592,7 +2594,8 @@ int aupat_get_buflocal_nr(const char *pat, int patlen)
 
     // "<buffer=123>"
     if (skipdigits(pat + 8) == pat + patlen - 1) {
-      return atoi(pat + 8);
+      char *p = (char *)pat + 8;
+      return getdigits_int(&p, false, 0);
     }
   }
 
diff --git a/src/nvim/errors.h b/src/nvim/errors.h
index 1ad98f2aeb..9fdb6a6895 100644
--- a/src/nvim/errors.h
+++ b/src/nvim/errors.h
@@ -18,6 +18,7 @@ EXTERN const char e_cmdwin[] INIT(= N_("E11: Invalid in command-line window; <CR
 EXTERN const char e_curdir[] INIT(= N_("E12: Command not allowed in secure mode in current dir or tag search"));
 EXTERN const char e_invalid_buffer_name_str[] INIT(= N_("E158: Invalid buffer name: %s"));
 EXTERN const char e_command_too_recursive[] INIT(= N_("E169: Command too recursive"));
+EXTERN const char e_buffer_nr_invalid_buffer_number[] INIT(= N_("E680: <buffer=%d>: invalid buffer number"));
 EXTERN const char e_buffer_is_not_loaded[] INIT(= N_("E681: Buffer is not loaded"));
 EXTERN const char e_endif[] INIT(= N_("E171: Missing :endif"));
 EXTERN const char e_endtry[] INIT(= N_("E600: Missing :endtry"));