diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 22c18d8..9731094 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2']
+        php: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
     name: PHP ${{ matrix.php }}
     steps:
     - name: Checkout
diff --git a/src/RRule.php b/src/RRule.php
index 8380c41..6602872 100755
--- a/src/RRule.php
+++ b/src/RRule.php
@@ -219,34 +219,38 @@ class RRule implements RRuleInterface
 		$this->rule = $parts; // save original rule
 
 		// WKST
-		$parts['WKST'] = strtoupper($parts['WKST']);
-		if (! array_key_exists($parts['WKST'], self::WEEKDAYS)) {
+		if (is_string($parts['WKST'])) {
+			$parts['WKST'] = strtoupper($parts['WKST']);
+			if (array_key_exists($parts['WKST'], self::WEEKDAYS)) {
+				$this->wkst = self::WEEKDAYS[$parts['WKST']];
+			}
+		}
+
+		if (!$this->wkst) {
 			throw new \InvalidArgumentException(
 				'The WKST rule part must be one of the following: '
 				.implode(', ',array_keys(self::WEEKDAYS))
 			);
 		}
-		$this->wkst = self::WEEKDAYS[$parts['WKST']];
 
 		// FREQ
 		if (is_integer($parts['FREQ'])) {
-			if ($parts['FREQ'] > self::SECONDLY || $parts['FREQ'] < self::YEARLY) {
-				throw new \InvalidArgumentException(
-					'The FREQ rule part must be one of the following: '
-					.implode(', ',array_keys(self::FREQUENCIES))
-				);
+			if ($parts['FREQ'] <= self::SECONDLY && $parts['FREQ'] >= self::YEARLY) {
+				$this->freq = $parts['FREQ'];
 			}
-			$this->freq = $parts['FREQ'];
 		}
-		else { // string
+		elseif (is_string($parts['FREQ'])) {
 			$parts['FREQ'] = strtoupper($parts['FREQ']);
-			if (! array_key_exists($parts['FREQ'], self::FREQUENCIES)) {
-				throw new \InvalidArgumentException(
-					'The FREQ rule part must be one of the following: '
-					.implode(', ',array_keys(self::FREQUENCIES))
-				);
+			if (array_key_exists($parts['FREQ'], self::FREQUENCIES)) {
+				$this->freq = self::FREQUENCIES[$parts['FREQ']];
 			}
-			$this->freq = self::FREQUENCIES[$parts['FREQ']];
+		} 
+
+		if (!$this->freq) {
+			throw new \InvalidArgumentException(
+				'The FREQ rule part must be one of the following: '
+				.implode(', ',array_keys(self::FREQUENCIES))
+			);
 		}
 
 		// INTERVAL
diff --git a/tests/RRuleTest.php b/tests/RRuleTest.php
index 9be5aa0..50e49ae 100644
--- a/tests/RRuleTest.php
+++ b/tests/RRuleTest.php
@@ -20,8 +20,12 @@ class RRuleTest extends TestCase
 			array(array()),
 			array(array('FOOBAR' => 'DAILY')),
 
-			array(array('FREQ' => 'foobar')),
+			'invalid string freq' => [['FREQ' => 'foobar']],
 			'Invalid integer frequency' => [['FREQ' => 42]],
+			'Array freq' => [['FREQ' => array()]],
+			'null freq' => [['FREQ' => null]],
+			'object freq' => [['FREQ' => new Stdclass()]],
+
 			array(array('FREQ' => 'DAILY', 'INTERVAL' => -1)),
 			array(array('FREQ' => 'DAILY', 'INTERVAL' => 1.5)),
 			array(array('FREQ' => 'DAILY', 'UNTIL' => 'foobar')),
@@ -87,6 +91,9 @@ class RRuleTest extends TestCase
 			array(array('FREQ' => 'MONTHLY', 'BYSECOND' => 61)),
 
 			'Invalid WKST' => [['FREQ' => 'DAILY', 'WKST' => 'XX']],
+			'Null WKST' => [['FREQ' => 'DAILY', 'WKST' => null]],
+			'Array WKST' => [['FREQ' => 'DAILY', 'WKST' => array()]],
+			'Object WKST' => [['FREQ' => 'DAILY', 'WKST' => new stdClass()]],
 
 			'Invalid DTSTART (invalid date)' => [['FREQ' => 'DAILY', 'DTSTART' => new stdClass()]]
 		);