diff --git a/mailpoet/lib/Validator/Schema.php b/mailpoet/lib/Validator/Schema.php
index e2407cb4b9..eb1dff3fa6 100644
--- a/mailpoet/lib/Validator/Schema.php
+++ b/mailpoet/lib/Validator/Schema.php
@@ -87,4 +87,12 @@ abstract class Schema {
   protected function getReservedKeywords(): array {
     return rest_get_allowed_schema_keywords();
   }
+
+  protected function validatePattern(string $pattern): void {
+    $escaped = str_replace('#', '\\#', $pattern);
+    $regex = "#$escaped#u";
+    if (@preg_match($regex, '') === false) {
+      throw new InvalidStateException("Invalid regular expression '$regex'");
+    }
+  }
 }
diff --git a/mailpoet/lib/Validator/Schema/ObjectSchema.php b/mailpoet/lib/Validator/Schema/ObjectSchema.php
index 8863b417a0..dd8cab7f7f 100644
--- a/mailpoet/lib/Validator/Schema/ObjectSchema.php
+++ b/mailpoet/lib/Validator/Schema/ObjectSchema.php
@@ -35,12 +35,12 @@ class ObjectSchema extends Schema {
    * @param array<string, Schema> $properties
    */
   public function patternProperties(array $properties): self {
-    return $this->updateSchemaProperty('patternProperties', array_map(
-      function (Schema $property) {
-        return $property->toArray();
-      },
-      $properties
-    ));
+    $patternProperties = [];
+    foreach ($properties as $key => $value) {
+      $this->validatePattern($key);
+      $patternProperties[$key] = $value->toArray();
+    }
+    return $this->updateSchemaProperty('patternProperties', $patternProperties);
   }
 
   public function minProperties(int $value): self {
diff --git a/mailpoet/lib/Validator/Schema/StringSchema.php b/mailpoet/lib/Validator/Schema/StringSchema.php
index a661a89184..172eb66e75 100644
--- a/mailpoet/lib/Validator/Schema/StringSchema.php
+++ b/mailpoet/lib/Validator/Schema/StringSchema.php
@@ -23,6 +23,7 @@ class StringSchema extends Schema {
    * See: https://developer.wordpress.org/rest-api/extending-the-rest-api/schema/#pattern
    */
   public function pattern(string $pattern): self {
+    $this->validatePattern($pattern);
     return $this->updateSchemaProperty('pattern', $pattern);
   }
 
diff --git a/mailpoet/tests/unit/Validator/Schema/ObjectSchemaTest.php b/mailpoet/tests/unit/Validator/Schema/ObjectSchemaTest.php
index 7ebdf73491..85fb3f35e8 100644
--- a/mailpoet/tests/unit/Validator/Schema/ObjectSchemaTest.php
+++ b/mailpoet/tests/unit/Validator/Schema/ObjectSchemaTest.php
@@ -2,6 +2,7 @@
 
 namespace MailPoet\Validator\Schema;
 
+use MailPoet\InvalidStateException;
 use MailPoet\Validator\Schema;
 use MailPoetUnitTest;
 
@@ -104,6 +105,11 @@ class ObjectSchemaTest extends MailPoetUnitTest {
       '{"type":"object","patternProperties":{"^number-[0-9]+":{"type":"number"},"^string-[0-9]+":{"type":"string"}}}',
       $object->toString()
     );
+
+    $this->assertInvalidPatternProperty(['\\' => $this->getNumberSchemaMock()], "Invalid regular expression '#\\#u'");
+    $this->assertInvalidPatternProperty(['\\#' => $this->getStringSchemaMock()], "Invalid regular expression '#\\\\##u'");
+    $this->assertInvalidPatternProperty(['[' => $this->getNumberSchemaMock()], "Invalid regular expression '#[#u'");
+    $this->assertInvalidPatternProperty(['[0-9' => $this->getStringSchemaMock()], "Invalid regular expression '#[0-9#u'");
   }
 
   public function testMinProperties(): void {
@@ -174,4 +180,15 @@ class ObjectSchemaTest extends MailPoetUnitTest {
       protected $schema = ['type' => 'string'];
     };
   }
+
+  private function assertInvalidPatternProperty(array $properties, string $message): void {
+    try {
+      (new ObjectSchema())->patternProperties($properties);
+    } catch (InvalidStateException $e) {
+      $this->assertSame($message, $e->getMessage());
+      return;
+    }
+    $class = InvalidStateException::class;
+    $this->fail("Exception '$class' with message '$message' was not thrown.");
+  }
 }
diff --git a/mailpoet/tests/unit/Validator/Schema/StringSchemaTest.php b/mailpoet/tests/unit/Validator/Schema/StringSchemaTest.php
index 50f838c615..3ee066e894 100644
--- a/mailpoet/tests/unit/Validator/Schema/StringSchemaTest.php
+++ b/mailpoet/tests/unit/Validator/Schema/StringSchemaTest.php
@@ -2,6 +2,7 @@
 
 namespace MailPoet\Validator\Schema;
 
+use MailPoet\InvalidStateException;
 use MailPoetUnitTest;
 
 class StringSchemaTest extends MailPoetUnitTest {
@@ -27,6 +28,11 @@ class StringSchemaTest extends MailPoetUnitTest {
     $string = (new StringSchema())->pattern('[0-9]+');
     $this->assertSame(['type' => 'string', 'pattern' => '[0-9]+'], $string->toArray());
     $this->assertSame('{"type":"string","pattern":"[0-9]+"}', $string->toString());
+
+    $this->assertInvalidPattern('\\', "Invalid regular expression '#\\#u'");
+    $this->assertInvalidPattern('\\#', "Invalid regular expression '#\\\\##u'");
+    $this->assertInvalidPattern('[', "Invalid regular expression '#[#u'");
+    $this->assertInvalidPattern('[0-9', "Invalid regular expression '#[0-9#u'");
   }
 
   public function testFormat(): void {
@@ -88,4 +94,15 @@ class StringSchemaTest extends MailPoetUnitTest {
     $this->assertNotSame($string->formatUri(), $string);
     $this->assertNotSame($string->formatUuid(), $string);
   }
+
+  private function assertInvalidPattern(string $pattern, string $message): void {
+    try {
+      (new StringSchema())->pattern($pattern);
+    } catch (InvalidStateException $e) {
+      $this->assertSame($message, $e->getMessage());
+      return;
+    }
+    $class = InvalidStateException::class;
+    $this->fail("Exception '$class' with message '$message' was not thrown.");
+  }
 }