diff --git a/lua/gitsigns/watcher.lua b/lua/gitsigns/watcher.lua
index 937e4710..71071b7c 100644
--- a/lua/gitsigns/watcher.lua
+++ b/lua/gitsigns/watcher.lua
@@ -116,6 +116,9 @@ function M.watch_gitdir(bufnr, gitdir)
 
     local info = string.format("Git dir update: '%s' %s", filename, inspect(events))
 
+    -- The luv docs say filename is passed as a string but it has been observed
+    -- to sometimes be nil.
+    --    https://github.com/lewis6991/gitsigns.nvim/issues/848
     if filename == nil or vim.endswith(filename, '.lock') then
       dprintf('%s (ignoring)', info)
       return