diff --git a/src/df.c b/src/df.c
index b803fc73b..8a0293ca9 100644
--- a/src/df.c
+++ b/src/df.c
@@ -127,6 +127,7 @@ static bool print_grand_total;
 
 /* Grand total data.  */
 static struct fs_usage grand_fsu;
+static bool grand_fsu_fsu_files_top_bit_set;
 
 /* Display modes.  */
 enum
@@ -993,8 +994,11 @@ get_field_values (struct field_values_t *bv,
 static void
 add_to_grand_total (struct field_values_t *bv, struct field_values_t *iv)
 {
-  if (known_value (iv->total))
-    grand_fsu.fsu_files += iv->total;
+  if (known_value (iv->total) && known_value (iv->available_to_root))
+    add_uint_with_neg_flag (&grand_fsu.fsu_files,
+                            &grand_fsu_fsu_files_top_bit_set,
+                            iv->total - iv->available_to_root,
+                            iv->total < iv->available_to_root);
   if (known_value (iv->available))
     grand_fsu.fsu_ffree += iv->available;
 
@@ -1860,6 +1864,9 @@ main (int argc, char **argv)
                  NULL, NULL, NULL, false, false, &grand_fsu, false);
 
       print_table ();
+
+      if (print_grand_total & grand_fsu_fsu_files_top_bit_set)
+        die (EXIT_FAILURE, 0, "iused < 0");
     }
   else
     {