--- a/aclocal.in
+++ b/aclocal.in
@@ -309,6 +309,12 @@ sub scan_m4_dirs ($@)
 
   foreach my $m4dir (@dirlist)
     {
+      if (! -d $m4dir)
+      {
+        msg ('override', "warning: skipping not existing directory `$m4dir'");
+        next;
+      }
+
       if (! opendir (DIR, $m4dir))
 	{
 	  fatal "couldn't open directory `$m4dir': $!";