diff --git a/system/Validation/FormatRules.php b/system/Validation/FormatRules.php
index 502f3ae09c82..af075a31e713 100644
--- a/system/Validation/FormatRules.php
+++ b/system/Validation/FormatRules.php
@@ -284,7 +284,13 @@ public function valid_base64($str = null): bool
             $str = (string) $str;
         }
 
-        return base64_encode(base64_decode($str, true)) === $str;
+        $decoded = base64_decode($str, true);
+
+        if ($decoded === false) {
+            return false;
+        }
+
+        return base64_encode($decoded) === $str;
     }
 
     /**
diff --git a/tests/system/Validation/FormatRulesTest.php b/tests/system/Validation/FormatRulesTest.php
index 7849ad3afd4c..0b4dc2151d9e 100644
--- a/tests/system/Validation/FormatRulesTest.php
+++ b/tests/system/Validation/FormatRulesTest.php
@@ -1053,6 +1053,10 @@ public static function provideBase64(): iterable
                 'FA08GG',
                 false,
             ],
+            [
+                'dGVszdA==',
+                false,
+            ],
             [
                 null,
                 false,
diff --git a/user_guide_src/source/changelogs/v4.6.4.rst b/user_guide_src/source/changelogs/v4.6.4.rst
index 116c371d10c2..98dba2bff6d5 100644
--- a/user_guide_src/source/changelogs/v4.6.4.rst
+++ b/user_guide_src/source/changelogs/v4.6.4.rst
@@ -44,6 +44,7 @@ Bugs Fixed
 - **Forge:** Fixed a bug in ``Postgre`` and ``SQLSRV`` where changing a column's default value using ``Forge::modifyColumn()`` method produced incorrect SQL syntax.
 - **Model:** Fixed a bug in ``Model::replace()`` where ``created_at`` field (when available) wasn't set correctly.
 - **Model:** Fixed a bug in ``Model::insertBatch()`` and ``Model::updateBatch()`` where casts were not applied to inserted or updated values.
+- **Validation:** Fixed a bug in the ``FormatRules::valid_base64()`` validation rule that caused a TypeError when checking invalid base64 strings.
 
 See the repo's
 `CHANGELOG.md <https://github.com/codeigniter4/CodeIgniter4/blob/develop/CHANGELOG.md>`_