diff --git a/src/Core/Parser/SyntaxTree/ObjectAccessorNode.php b/src/Core/Parser/SyntaxTree/ObjectAccessorNode.php
index 0818b692a..3c4420bc3 100644
--- a/src/Core/Parser/SyntaxTree/ObjectAccessorNode.php
+++ b/src/Core/Parser/SyntaxTree/ObjectAccessorNode.php
@@ -11,6 +11,7 @@
 
 use TYPO3Fluid\Fluid\Core\Compiler\TemplateCompiler;
 use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;
+use TYPO3Fluid\Fluid\Core\Variables\InvalidVariableIdentifierException;
 
 /**
  * A node which handles object access. This means it handles structures like {object.accessor.bla}
@@ -34,6 +35,12 @@ final class ObjectAccessorNode extends AbstractNode
      */
     public function __construct(string $objectPath)
     {
+        // TODO consider removing this exception in later Fluid versions. For now it allows the
+        //      TemplateValidator to find occurrances of variables starting with "_" in templates,
+        //      which are no longer allowed since Fluid 5.
+        if (str_starts_with($objectPath, '_') && $objectPath !== '_all') {
+            throw new InvalidVariableIdentifierException('Variable identifiers cannot start with a "_": ' . $objectPath, 1765900762);
+        }
         $this->objectPath = $objectPath;
     }
 
diff --git a/tests/Functional/Fixtures/Validation/AccessToInvalidVariableName.fluid.html b/tests/Functional/Fixtures/Validation/AccessToInvalidVariableName.fluid.html
new file mode 100644
index 000000000..04b2022b7
--- /dev/null
+++ b/tests/Functional/Fixtures/Validation/AccessToInvalidVariableName.fluid.html
@@ -0,0 +1 @@
+{_something.sub}
diff --git a/tests/Functional/Validation/TemplateValidatorTest.php b/tests/Functional/Validation/TemplateValidatorTest.php
index 2fca36b6d..816bf6df1 100644
--- a/tests/Functional/Validation/TemplateValidatorTest.php
+++ b/tests/Functional/Validation/TemplateValidatorTest.php
@@ -26,6 +26,7 @@ public static function validatorFindsParseErrorsDataProvider(): array
             [$fixturePath . 'InvalidNamespace.fluid.html', 'Unknown Namespace: foo', 0],
             [$fixturePath . 'RequiredArgumentMissing.fluid.html', 'Required argument "each" was not supplied.', 1237823699],
             [$fixturePath . 'RedefinedComponentArgument.fluid.html', 'Template argument "foo" has been defined multiple times.', 1744908509],
+            [$fixturePath . 'AccessToInvalidVariableName.fluid.html', 'Variable identifiers cannot start with a "_": _something.sub', 1765900762],
         ];
     }