diff --git a/src/Faker/Provider/cs_CZ/Person.php b/src/Faker/Provider/cs_CZ/Person.php
index 6e2bab1375..64b4e9e0c8 100644
--- a/src/Faker/Provider/cs_CZ/Person.php
+++ b/src/Faker/Provider/cs_CZ/Person.php
@@ -438,8 +438,8 @@ public function birthNumber($gender = null, $minAge = 0, $maxAge = 100, $slashPr
             $gender = $this->generator->boolean() ? static::GENDER_MALE : static::GENDER_FEMALE;
         }
 
-        $startTimestamp = strtotime("-${maxAge} year");
-        $endTimestamp = strtotime("-${minAge} year");
+        $startTimestamp = strtotime(sprintf('-%d year', $maxAge));
+        $endTimestamp = strtotime(sprintf('-%d year', $minAge));
         $randTimestamp = self::numberBetween($startTimestamp, $endTimestamp);
 
         $year = (int) (date('Y', $randTimestamp));