diff --git a/src/main/java/com/github/cleanddd/model/Course.java b/src/main/java/com/github/cleanddd/model/Course.java
index 71232c7..70180d3 100644
--- a/src/main/java/com/github/cleanddd/model/Course.java
+++ b/src/main/java/com/github/cleanddd/model/Course.java
@@ -34,7 +34,7 @@ public class Course {
     }
 
     public Course enrollStudent() {
-        return newCourse().numberOfStudents(numberOfStudents.get() + 1).build();
+        return newCourse().numberOfStudents(numberOfStudents.incrementAndGet()).build();
     }
 
     private CourseBuilder newCourse() {
diff --git a/src/main/java/com/github/cleanddd/usecase/EnrollStudentUseCase.java b/src/main/java/com/github/cleanddd/usecase/EnrollStudentUseCase.java
index 8bda915..1918fb2 100644
--- a/src/main/java/com/github/cleanddd/usecase/EnrollStudentUseCase.java
+++ b/src/main/java/com/github/cleanddd/usecase/EnrollStudentUseCase.java
@@ -72,7 +72,8 @@ public class EnrollStudentUseCase implements EnrollStudentInputPort {
                 persistenceOps.persist(enrollResult.getStudent());
 
                 final Course course = persistenceOps.obtainCourseById(courseId);
-                persistenceOps.persist(course.enrollStudent());
+                final Course updatedCourse = course.enrollStudent();
+                persistenceOps.persist(updatedCourse);
             }
 
             // present the result of enrollment