diff --git a/block.c b/block.c
index ba67c0d..50dab8e 100644
--- a/block.c
+++ b/block.c
@@ -2800,7 +2800,9 @@
          *
          * [sector_num+x, nr_sectors] allocated.
          */
-        if (n > pnum_inter) {
+        if (n > pnum_inter &&
+            (intermediate == top ||
+             sector_num + pnum_inter < intermediate->total_sectors)) {
             n = pnum_inter;
         }