diff --git a/src/Transactions/TransactionLineFields/PeriodField.php b/src/Transactions/TransactionLineFields/PeriodField.php
index 13fe4357..347c7a65 100644
--- a/src/Transactions/TransactionLineFields/PeriodField.php
+++ b/src/Transactions/TransactionLineFields/PeriodField.php
@@ -24,9 +24,9 @@ public function getPeriod(): ?string
      * @param string $period
      * @return $this
      */
-    public function setPeriod(string $period): self
+    public function setPeriod(?string $period): self
     {
-        if (!preg_match("!\\d{4}/\\d{1,2}!", $period)) {
+        if ($period !== null && !preg_match("!\\d{4}/\\d{1,2}!", $period)) {
             throw new \InvalidArgumentException("Period must be in YYYY/PP format (got: {$period}.");
         }
 
diff --git a/tests/UnitTests/Transactions/TransactionLineFields/PeriodFieldUnitTest.php b/tests/UnitTests/Transactions/TransactionLineFields/PeriodFieldUnitTest.php
new file mode 100644
index 00000000..165b3691
--- /dev/null
+++ b/tests/UnitTests/Transactions/TransactionLineFields/PeriodFieldUnitTest.php
@@ -0,0 +1,40 @@
+<?php
+
+use PhpTwinfield\Transactions\TransactionLineFields\PeriodField;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @covers \PhpTwinfield\Transactions\TransactionLineFields\PeriodField
+ */
+class PeriodFieldUnitTest extends TestCase
+{
+    /** @test */
+    public function testPeriodInCorrectFormat()
+    {
+        $periodFieldTrait = $this->getMockForTrait(PeriodField::class);
+
+        $periodFieldTrait->setPeriod('2020/01');
+
+        $this->assertEquals('2020/01', $periodFieldTrait->getPeriod());
+    }
+
+    /** @test */
+    public function testPeriodCanBeNull()
+    {
+        $periodFieldTrait = $this->getMockForTrait(PeriodField::class);
+
+        $periodFieldTrait->setPeriod(null);
+
+        $this->assertNull($periodFieldTrait->getPeriod());
+    }
+
+    /** @test */
+    public function testPeriodInIncorrectFormatThrowsException()
+    {
+        $this->expectException(\InvalidArgumentException::class);
+
+        $periodFieldTrait = $this->getMockForTrait(PeriodField::class);
+
+        $periodFieldTrait->setPeriod('asdf');
+    }
+}