diff --git a/system/Language/Language.php b/system/Language/Language.php
index 70d109e2319b..32eee39f2a13 100644
--- a/system/Language/Language.php
+++ b/system/Language/Language.php
@@ -12,6 +12,7 @@
 namespace CodeIgniter\Language;
 
 use Config\Services;
+use InvalidArgumentException;
 use MessageFormatter;
 
 /**
@@ -191,7 +192,14 @@ protected function formatMessage($message, array $args = [])
             return $message;
         }
 
-        return MessageFormatter::formatMessage($this->locale, $message, $args);
+        $formatted = MessageFormatter::formatMessage($this->locale, $message, $args);
+        if ($formatted === false) {
+            throw new InvalidArgumentException(
+                lang('Language.invalidMessageFormat', [$message, implode(',', $args)])
+            );
+        }
+
+        return $formatted;
     }
 
     /**
diff --git a/system/Language/en/Language.php b/system/Language/en/Language.php
new file mode 100644
index 000000000000..80920b2f16d5
--- /dev/null
+++ b/system/Language/en/Language.php
@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * This file is part of CodeIgniter 4 framework.
+ *
+ * (c) CodeIgniter Foundation <admin@codeigniter.com>
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+// "Language" language settings
+return [
+    'invalidMessageFormat' => 'Invalid message format: "{0}", args: "{1}"',
+];
diff --git a/tests/system/Language/LanguageTest.php b/tests/system/Language/LanguageTest.php
index 34313e8dfac8..5c75a6d9a28f 100644
--- a/tests/system/Language/LanguageTest.php
+++ b/tests/system/Language/LanguageTest.php
@@ -14,6 +14,7 @@
 use CodeIgniter\Test\CIUnitTestCase;
 use CodeIgniter\Test\Mock\MockLanguage;
 use Config\Services;
+use InvalidArgumentException;
 use MessageFormatter;
 use Tests\Support\Language\SecondMockLanguage;
 
@@ -126,6 +127,28 @@ public function testGetLineArrayFormatsMessages(): void
         $this->assertSame(['45 related books.'], $this->lang->getLine('books.bookList', [91 / 2]));
     }
 
+    /**
+     * @see https://github.com/codeigniter4/shield/issues/851
+     */
+    public function testGetLineInvalidFormatMessage(): void
+    {
+        // No intl extension? then we can't test this - go away....
+        if (! class_exists(MessageFormatter::class)) {
+            $this->markTestSkipped('No intl support.');
+        }
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage(
+            'Invalid message format: "تم الكشف عن كلمة المرور {0} بسبب اختراق البيانات وشوهدت {1 ، عدد} مرة في {2} في كلمات المرور المخترقة.", args: "password,hits,wording"'
+        );
+
+        $this->lang->setData('Auth', [
+            'errorPasswordPwned' => 'تم الكشف عن كلمة المرور {0} بسبب اختراق البيانات وشوهدت {1 ، عدد} مرة في {2} في كلمات المرور المخترقة.',
+        ]);
+
+        $this->lang->getLine('Auth.errorPasswordPwned', ['password', 'hits', 'wording']);
+    }
+
     /**
      * @see https://github.com/codeigniter4/CodeIgniter4/issues/891
      */