diff --git a/src/treelearner/monotone_constraints.hpp b/src/treelearner/monotone_constraints.hpp
index fbc8f2e6a933..dcad0d6d3288 100644
--- a/src/treelearner/monotone_constraints.hpp
+++ b/src/treelearner/monotone_constraints.hpp
@@ -78,8 +78,8 @@ class LeafConstraintsBase {
     tree_ = tree;
   }
 
-  private:
-    const Tree* tree_;
+ private:
+  const Tree* tree_;
 };
 
 class BasicLeafConstraints : public LeafConstraintsBase {