diff --git a/block_int.h b/block_int.h
index e3bfd192a2..b64a0095f5 100644
--- a/block_int.h
+++ b/block_int.h
@@ -224,7 +224,9 @@ static inline unsigned int get_physical_block_exp(BlockConf *conf)
 {
     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++;
     }