diff --git a/src/Type/DatePeriod.php b/src/Type/DatePeriod.php
index d25985c..8556031 100644
--- a/src/Type/DatePeriod.php
+++ b/src/Type/DatePeriod.php
@@ -114,7 +114,8 @@ class DatePeriod extends BaseType
      * Returns formatted one of the period's date: start date or end date
      *
      * @param string $format    Format used to format the date
-     * @param bool   $startDate (optional) If is set to true, start date will be formatted. Otherwise - end date.
+     * @param bool   $startDate (optional) If is set to true, start date will be formatted (default behaviour).
+     *                          Otherwise - end date.
      * @return string
      */
     public function getFormattedDate(string $format, bool $startDate = true): string
diff --git a/tests/Type/DatePeriodTest.php b/tests/Type/DatePeriodTest.php
index 0069a3c..c6ae1f8 100644
--- a/tests/Type/DatePeriodTest.php
+++ b/tests/Type/DatePeriodTest.php
@@ -77,6 +77,18 @@ class DatePeriodTest extends BaseTypeTestCase
         self::assertEquals('', $period->getFormattedDate($format));
     }
 
+    /**
+     * @param DatePeriod $period    The date period to verify
+     * @param string     $format    Format used to format the date
+     * @param bool       $startDate If is set to true, start date is formatted. Otherwise - end date.
+     *
+     * @dataProvider provideDatePeriodAndUnknownDate
+     */
+    public function testGetFormattedDateUsingUnknownDate(DatePeriod $period, $format, $startDate): void
+    {
+        self::assertEquals('', $period->getFormattedDate($format, $startDate));
+    }
+
     /**
      * @param DatePeriod $period   The date period to verify
      * @param string     $format   Format used to format the date
@@ -156,6 +168,35 @@ class DatePeriodTest extends BaseTypeTestCase
         ];
     }
 
+    public function provideDatePeriodAndUnknownDate(): ?Generator
+    {
+        $date = new DateTime('2001-01-01');
+
+        yield[
+            new DatePeriod(),
+            'Y-m-d',
+            false,
+        ];
+
+        yield[
+            new DatePeriod(),
+            'Y-m-d',
+            true,
+        ];
+
+        yield[
+            new DatePeriod($date),
+            'Y-m-d',
+            false,
+        ];
+
+        yield[
+            new DatePeriod(null, $date),
+            'Y-m-d',
+            true,
+        ];
+    }
+
     /**
      * Provides period and format of date to verify using the start date
      *