diff --git a/app/src/main/java/org/cutem/cutecalendar/util/CalendarUtil.java b/app/src/main/java/org/cutem/cutecalendar/util/CalendarUtil.java
index 6a7c7a0..1c3ffd6 100644
--- a/app/src/main/java/org/cutem/cutecalendar/util/CalendarUtil.java
+++ b/app/src/main/java/org/cutem/cutecalendar/util/CalendarUtil.java
@@ -1,7 +1,6 @@
 package org.cutem.cutecalendar.util;
 
 import org.jetbrains.annotations.NotNull;
-import org.jetbrains.annotations.Nullable;
 
 import java.util.Calendar;
 
@@ -186,13 +185,14 @@ public class CalendarUtil {
      * Returns a calendar where each field is designated by the corresponding string.
      *
      * @param calendarString the string that matches <em>"yyyy-mm-dd/hh:mm"</em>
-     * @return a {@code Calendar} instance if date is valid; {@code null} otherwise
+     * @return a {@code Calendar} instance
+     * @throws IllegalArgumentException if the only argument is bad formatted
      * @see #isValidDateString(String)
      */
-    @Nullable
+    @NotNull
     public static Calendar constructCalendar(@NotNull String calendarString) {
         if (!isValidCalendarString(calendarString)) {
-            return null;
+            throw new IllegalArgumentException("bad formation");
         }
         String[] cal = calendarString.split("/");
         return constructCalendar(cal[0], cal[1]);
@@ -203,14 +203,15 @@ public class CalendarUtil {
      *
      * @param date the string that matches <em>"yyyy-mm-dd"</em>
      * @param time the string that matched <em>"hh:mm"</em>
-     * @return a {@code Calendar} instance if both arguments are valid; {@code null} otherwise
+     * @return a {@code Calendar} instance
+     * @throws IllegalArgumentException if one of arguments is bad formatted
      * @see #isValidDateString(String)
      * @see #isValidTimeString(String)
      */
-    @Nullable
+    @NotNull
     public static Calendar constructCalendar(@NotNull String date, @NotNull String time) {
         if (!isValidDateString(date) || !isValidTimeString(time)) {
-            return null;
+            throw new IllegalArgumentException("bad formation");
         }
         String[] slices = date.split("-");
         int y = Integer.parseInt(slices[0]);
diff --git a/app/src/test/java/org/cutem/cutecalendar/util/CalendarUtilTest.java b/app/src/test/java/org/cutem/cutecalendar/util/CalendarUtilTest.java
index 67ca010..a2f392b 100644
--- a/app/src/test/java/org/cutem/cutecalendar/util/CalendarUtilTest.java
+++ b/app/src/test/java/org/cutem/cutecalendar/util/CalendarUtilTest.java
@@ -125,8 +125,13 @@ public class CalendarUtilTest {
         exp = new int[]{2018, 5, 2, 11, 30, 0, 0};
         assertArrayEquals(exp, res);
 
-        cal = CalendarUtil.constructCalendar(s3 + '/' + s2);
-        assertNull(cal);
+        boolean ok = false;
+        try {
+            CalendarUtil.constructCalendar(s3 + '/' + s2);
+        } catch (IllegalArgumentException e) {
+            ok = true;
+        }
+        assertTrue(ok);
     }
 
 }
\ No newline at end of file