diff --git a/include/xgboost/host_device_vector.h b/include/xgboost/host_device_vector.h
index 75595187ef86..5bcceddcd9ca 100644
--- a/include/xgboost/host_device_vector.h
+++ b/include/xgboost/host_device_vector.h
@@ -95,6 +95,7 @@ class HostDeviceVector {
   HostDeviceVector<T>& operator=(const HostDeviceVector<T>&) = delete;
   HostDeviceVector<T>& operator=(HostDeviceVector<T>&&);
 
+  bool Empty() const { return Size() == 0; }
   size_t Size() const;
   int DeviceIdx() const;
   common::Span<T> DeviceSpan();
diff --git a/src/common/host_device_vector.cu b/src/common/host_device_vector.cu
index 3ffcf757e846..6afdadf39322 100644
--- a/src/common/host_device_vector.cu
+++ b/src/common/host_device_vector.cu
@@ -59,7 +59,9 @@ class HostDeviceVectorImpl {
     }
   }
 
-  size_t Size() const { return HostCanRead() ? data_h_.size() : data_d_->size(); }
+  size_t Size() const {
+    return HostCanRead() ? data_h_.size() : data_d_ ? data_d_->size() : 0;
+  }
 
   int DeviceIdx() const { return device_; }
 
diff --git a/tests/cpp/common/test_host_device_vector.cu b/tests/cpp/common/test_host_device_vector.cu
index e5ac339e794f..f380385852d5 100644
--- a/tests/cpp/common/test_host_device_vector.cu
+++ b/tests/cpp/common/test_host_device_vector.cu
@@ -176,6 +176,13 @@ TEST(HostDeviceVector, Span) {
   ASSERT_TRUE(vec.HostCanWrite());
 }
 
+TEST(HostDeviceVector, Empty) {
+  HostDeviceVector<float> vec {1.0f, 2.0f, 3.0f, 4.0f};
+  HostDeviceVector<float> another { std::move(vec) };
+  ASSERT_FALSE(another.Empty());
+  ASSERT_TRUE(vec.Empty());
+}
+
 TEST(HostDeviceVector, MGPU_Basic) {  // NOLINT
   if (AllVisibleGPUs() < 2) {
     LOG(WARNING) << "Not testing in multi-gpu environment.";