diff --git a/src/nvim/bufwrite.c b/src/nvim/bufwrite.c
index 2993d14d20..2f692f77e1 100644
--- a/src/nvim/bufwrite.c
+++ b/src/nvim/bufwrite.c
@@ -698,7 +698,7 @@ char *buf_get_backup_name(char *fname, char **dirp, bool no_prepend_dot, char *b
       xfree(failed_dir);
     }
   }
-  if (after_pathsep(IObuff, p) && p[-1] == p[-2]) {
+  if (dir_len > 1 && after_pathsep(IObuff, p) && p[-1] == p[-2]) {
     // path ends with '//', use full path
     if ((p = make_percent_swname(IObuff, p, fname))
         != NULL) {
diff --git a/src/nvim/undo.c b/src/nvim/undo.c
index 4a4e0daba7..d6d443b0d8 100644
--- a/src/nvim/undo.c
+++ b/src/nvim/undo.c
@@ -712,7 +712,7 @@ char *u_get_undo_file_name(const char *const buf_ffname, const bool reading)
 
       // Remove trailing pathseps from directory name
       char *p = &dir_name[dir_len - 1];
-      while (vim_ispathsep(*p)) {
+      while (dir_len > 1 && vim_ispathsep(*p)) {
         *p-- = NUL;
         dir_len--;
       }