diff --git a/composer.json b/composer.json
index d77096c..fdb46ee 100644
--- a/composer.json
+++ b/composer.json
@@ -18,7 +18,6 @@
     "require-dev": {
         "phpstan/phpstan": "^1.0",
         "phpunit/phpunit": "^7.0|^8.0|^9.0",
-        "phpstan/phpstan-webmozart-assert": "^1.0",
         "phpstan/extension-installer": "^1.0"
     },
     "autoload": {
diff --git a/src/Cron/CronExpression.php b/src/Cron/CronExpression.php
index 7f0dab0..f3d8eb0 100644
--- a/src/Cron/CronExpression.php
+++ b/src/Cron/CronExpression.php
@@ -12,7 +12,6 @@
 use InvalidArgumentException;
 use LogicException;
 use RuntimeException;
-use Webmozart\Assert\Assert;
 
 /**
  * CRON expression parser that can determine whether or not a CRON expression is
@@ -200,7 +199,12 @@ public function __construct(string $expression, ?FieldFactoryInterface $fieldFac
     public function setExpression(string $value): CronExpression
     {
         $split = preg_split('/\s/', $value, -1, PREG_SPLIT_NO_EMPTY);
-        Assert::isArray($split);
+
+        if (!\is_array($split)) {
+            throw new InvalidArgumentException(
+                $value . ' is not a valid CRON expression'
+            );
+        }
 
         $notEnoughParts = \count($split) < 5;
 
@@ -334,7 +338,10 @@ public function getMultipleRunDates(int $total, $currentTime = 'now', bool $inve
             $currentTime = new DateTime($currentTime);
         }
 
-        Assert::isInstanceOf($currentTime, DateTime::class);
+        if (!$currentTime instanceof DateTime) {
+            throw new InvalidArgumentException('invalid current time');
+        }
+
         $currentTime->setTimezone(new DateTimeZone($timeZone));
 
         $matches = [];
@@ -420,7 +427,10 @@ public function isDue($currentTime = 'now', $timeZone = null): bool
             $currentTime = new DateTime($currentTime);
         }
 
-        Assert::isInstanceOf($currentTime, DateTime::class);
+        if (!$currentTime instanceof DateTime) {
+            throw new InvalidArgumentException('invalid current time');
+        }
+
         $currentTime->setTimezone(new DateTimeZone($timeZone));
 
         // drop the seconds to 0
@@ -462,7 +472,10 @@ protected function getRunDate($currentTime = null, int $nth = 0, bool $invert =
             $currentDate = new DateTime('now');
         }
 
-        Assert::isInstanceOf($currentDate, DateTime::class);
+        if (!$currentDate instanceof DateTime) {
+            throw new InvalidArgumentException('invalid current date');
+        }
+
         $currentDate->setTimezone(new DateTimeZone($timeZone));
         // Workaround for setTime causing an offset change: https://bugs.php.net/bug.php?id=81074
         $currentDate = DateTime::createFromFormat("!Y-m-d H:iO", $currentDate->format("Y-m-d H:iP"), $currentDate->getTimezone());