diff --git a/block_int.h b/block_int.h
index e3bfd19..b64a009 100644
--- a/block_int.h
+++ b/block_int.h
@@ -224,7 +224,9 @@
 {
     unsigned int exp = 0, size;
 
-    for (size = conf->physical_block_size; size > 512; size >>= 1) {
+    for (size = conf->physical_block_size;
+        size > conf->logical_block_size;
+        size >>= 1) {
         exp++;
     }