diff --git a/src/tree/hist/evaluate_splits.h b/src/tree/hist/evaluate_splits.h
index 0a09718ef..3c5299547 100644
--- a/src/tree/hist/evaluate_splits.h
+++ b/src/tree/hist/evaluate_splits.h
@@ -389,6 +389,7 @@ class HistEvaluator {
     tree_evaluator_.AddSplit(candidate.nid, left_child, right_child,
                              tree[candidate.nid].SplitIndex(), left_weight,
                              right_weight);
+    evaluator = tree_evaluator_.GetEvaluator();
 
     auto max_node = std::max(left_child, tree[candidate.nid].RightChild());
     max_node = std::max(candidate.nid, max_node);
diff --git a/src/tree/split_evaluator.h b/src/tree/split_evaluator.h
index d19755d37..b65c1d055 100644
--- a/src/tree/split_evaluator.h
+++ b/src/tree/split_evaluator.h
@@ -48,6 +48,8 @@ class TreeEvaluator {
       monotone_.HostVector().resize(n_features, 0);
       has_constraint_ = false;
     } else {
+      CHECK_LE(p.monotone_constraints.size(), n_features)
+          << "The size of monotone constraint should be less or equal to the number of features.";
       monotone_.HostVector() = p.monotone_constraints;
       monotone_.HostVector().resize(n_features, 0);
       // Initialised to some small size, can grow if needed
diff --git a/tests/cpp/tree/test_constraints.cc b/tests/cpp/tree/test_constraints.cc
index fa923a621..e6b36e797 100644
--- a/tests/cpp/tree/test_constraints.cc
+++ b/tests/cpp/tree/test_constraints.cc
@@ -6,6 +6,9 @@
 #include <string>
 
 #include "../../../src/tree/constraints.h"
+#include "../../../src/tree/hist/evaluate_splits.h"
+#include "../../../src/tree/hist/expand_entry.h"
+#include "../helpers.h"
 
 namespace xgboost {
 namespace tree {
@@ -56,5 +59,38 @@ TEST(CPUFeatureInteractionConstraint, Basic) {
   ASSERT_FALSE(constraints.Query(1, 5));
 }
 
+TEST(CPUMonoConstraint, Basic) {
+  std::size_t kRows{64}, kCols{16};
+  Context ctx;
+
+  TrainParam param;
+  std::vector<std::int32_t> mono(kCols, 1);
+  I32Array arr;
+  for (std::size_t i = 0; i < kCols; ++i) {
+    arr.GetArray().push_back(mono[i]);
+  }
+  Json jarr{std::move(arr)};
+  std::string str_mono;
+  Json::Dump(jarr, &str_mono);
+  str_mono.front() = '(';
+  str_mono.back() = ')';
+
+  param.UpdateAllowUnknown(Args{{"monotone_constraints", str_mono}});
+
+  auto Xy = RandomDataGenerator{kRows, kCols, 0.0}.GenerateDMatrix(true);
+  auto sampler = std::make_shared<common::ColumnSampler>();
+
+  HistEvaluator<CPUExpandEntry> evalutor{param, Xy->Info(), ctx.Threads(), sampler};
+  evalutor.InitRoot(GradStats{2.0, 2.0});
+
+  SplitEntry split;
+  split.Update(1.0f, 0, 3.0, false, false, GradStats{1.0, 1.0}, GradStats{1.0, 1.0});
+  CPUExpandEntry entry{0, 0, split};
+  RegTree tree;
+  tree.param.UpdateAllowUnknown(Args{{"num_feature", std::to_string(kCols)}});
+  evalutor.ApplyTreeSplit(entry, &tree);
+
+  ASSERT_TRUE(evalutor.Evaluator().has_constraint);
+}
 }  // namespace tree
 }  // namespace xgboost