diff --git a/src/main/java/io/spring/api/ArticleApi.java b/src/main/java/io/spring/api/ArticleApi.java
index c80afa31d..92b70fcac 100644
--- a/src/main/java/io/spring/api/ArticleApi.java
+++ b/src/main/java/io/spring/api/ArticleApi.java
@@ -33,7 +33,7 @@ public class ArticleApi {
   private ArticleCommandService articleCommandService;
 
   @GetMapping
-  public ResponseEntity<?> article(
+  public ResponseEntity<Map<String, Object>> article(
       @PathVariable("slug") String slug, @AuthenticationPrincipal User user) {
     return articleQueryService
         .findBySlug(slug, user)
@@ -42,7 +42,7 @@ public ResponseEntity<?> article(
   }
 
   @PutMapping
-  public ResponseEntity<?> updateArticle(
+  public ResponseEntity<Map<String, Object>> updateArticle(
       @PathVariable("slug") String slug,
       @AuthenticationPrincipal User user,
       @Valid @RequestBody UpdateArticleParam updateArticleParam) {
diff --git a/src/main/java/io/spring/api/CommentsApi.java b/src/main/java/io/spring/api/CommentsApi.java
index c5f7e77e9..6dd329f8e 100644
--- a/src/main/java/io/spring/api/CommentsApi.java
+++ b/src/main/java/io/spring/api/CommentsApi.java
@@ -38,7 +38,7 @@ public class CommentsApi {
   private CommentQueryService commentQueryService;
 
   @PostMapping
-  public ResponseEntity<?> createComment(
+  public ResponseEntity<Map<String, Object>> createComment(
       @PathVariable("slug") String slug,
       @AuthenticationPrincipal User user,
       @Valid @RequestBody NewCommentParam newCommentParam) {
diff --git a/src/main/java/io/spring/api/exception/InvalidRequestException.java b/src/main/java/io/spring/api/exception/InvalidRequestException.java
index 68b6c868a..7360d717e 100644
--- a/src/main/java/io/spring/api/exception/InvalidRequestException.java
+++ b/src/main/java/io/spring/api/exception/InvalidRequestException.java
@@ -4,7 +4,7 @@
 
 @SuppressWarnings("serial")
 public class InvalidRequestException extends RuntimeException {
-  private final Errors errors;
+  private final transient Errors errors;
 
   public InvalidRequestException(Errors errors) {
     super("");
diff --git a/src/main/java/io/spring/infrastructure/mybatis/DateTimeHandler.java b/src/main/java/io/spring/infrastructure/mybatis/DateTimeHandler.java
index 19323e565..9e65a63a3 100644
--- a/src/main/java/io/spring/infrastructure/mybatis/DateTimeHandler.java
+++ b/src/main/java/io/spring/infrastructure/mybatis/DateTimeHandler.java
@@ -15,30 +15,34 @@
 @MappedTypes(DateTime.class)
 public class DateTimeHandler implements TypeHandler<DateTime> {
 
-  private static final Calendar UTC_CALENDAR = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
+  private static final TimeZone UTC = TimeZone.getTimeZone("UTC");
+
+  private static Calendar utcCalendar() {
+    return Calendar.getInstance(UTC);
+  }
 
   @Override
   public void setParameter(PreparedStatement ps, int i, DateTime parameter, JdbcType jdbcType)
       throws SQLException {
     ps.setTimestamp(
-        i, parameter != null ? new Timestamp(parameter.getMillis()) : null, UTC_CALENDAR);
+        i, parameter != null ? new Timestamp(parameter.getMillis()) : null, utcCalendar());
   }
 
   @Override
   public DateTime getResult(ResultSet rs, String columnName) throws SQLException {
-    Timestamp timestamp = rs.getTimestamp(columnName, UTC_CALENDAR);
+    Timestamp timestamp = rs.getTimestamp(columnName, utcCalendar());
     return timestamp != null ? new DateTime(timestamp.getTime()) : null;
   }
 
   @Override
   public DateTime getResult(ResultSet rs, int columnIndex) throws SQLException {
-    Timestamp timestamp = rs.getTimestamp(columnIndex, UTC_CALENDAR);
+    Timestamp timestamp = rs.getTimestamp(columnIndex, utcCalendar());
     return timestamp != null ? new DateTime(timestamp.getTime()) : null;
   }
 
   @Override
   public DateTime getResult(CallableStatement cs, int columnIndex) throws SQLException {
-    Timestamp ts = cs.getTimestamp(columnIndex, UTC_CALENDAR);
+    Timestamp ts = cs.getTimestamp(columnIndex, utcCalendar());
     return ts != null ? new DateTime(ts.getTime()) : null;
   }
 }