diff --git a/src/partition.cc b/src/partition.cc
index 9c95ba31e..480199644 100644
--- a/src/partition.cc
+++ b/src/partition.cc
@@ -423,7 +423,7 @@ std::vector<SplitPair> Partition::split_blocks(
         do {
             // we choose the swapping strategy using XOR operation
             while((repr_marked 
-                  ^ !used_states[get_block_item(iter_first).state])) {
+                  ^ (!used_states[get_block_item(iter_first).state]))) {
                 // this visited state will be part of the former block
                 ++iter_first;
             }