diff --git a/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php b/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php
index 52d5cfe5e89..82cbcd94b46 100644
--- a/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php
+++ b/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php
@@ -7,7 +7,6 @@
 use PhpParser\Node;
 use PhpParser\Node\Arg;
 use PhpParser\Node\Expr;
-use PhpParser\Node\Expr\ArrayDimFetch;
 use PhpParser\Node\Expr\Cast\String_ as CastString_;
 use PhpParser\Node\Expr\ConstFetch;
 use PhpParser\Node\Expr\FuncCall;
@@ -17,7 +16,10 @@
 use PhpParser\Node\Scalar\String_;
 use PHPStan\Analyser\Scope;
 use PHPStan\Reflection\ClassReflection;
+use PHPStan\Reflection\FunctionReflection;
 use PHPStan\Reflection\Native\NativeFunctionReflection;
+use PHPStan\Reflection\Native\NativeParameterWithPhpDocsReflection;
+use PHPStan\Reflection\ParametersAcceptor;
 use PHPStan\Type\ErrorType;
 use PHPStan\Type\MixedType;
 use PHPStan\Type\NullType;
@@ -113,9 +115,23 @@ public function refactor(Node $node): ?Node
         $classReflection = $scope->getClassReflection();
         $isTrait = $classReflection instanceof ClassReflection && $classReflection->isTrait();
 
+        $functionReflection = $this->reflectionResolver->resolveFunctionLikeReflectionFromCall($node);
+        if (! $functionReflection instanceof FunctionReflection) {
+            return null;
+        }
+
+        $parametersAcceptor = ParametersAcceptorSelectorVariantsWrapper::select($functionReflection, $node, $scope);
         $isChanged = false;
+
         foreach ($positions as $position) {
-            $result = $this->processNullToStrictStringOnNodePosition($node, $args, $position, $isTrait, $scope);
+            $result = $this->processNullToStrictStringOnNodePosition(
+                $node,
+                $args,
+                $position,
+                $isTrait,
+                $scope,
+                $parametersAcceptor
+            );
             if ($result instanceof Node) {
                 $node = $result;
                 $isChanged = true;
@@ -167,7 +183,8 @@ private function processNullToStrictStringOnNodePosition(
         array $args,
         int|string $position,
         bool $isTrait,
-        Scope $scope
+        Scope $scope,
+        ParametersAcceptor $parametersAcceptor
     ): ?FuncCall {
         if (! isset($args[$position])) {
             return null;
@@ -208,8 +225,12 @@ private function processNullToStrictStringOnNodePosition(
             return null;
         }
 
-        if ($type instanceof MixedType && $args[$position]->value instanceof ArrayDimFetch) {
-            return null;
+        $parameter = $parametersAcceptor->getParameters()[$position] ?? null;
+        if ($parameter instanceof NativeParameterWithPhpDocsReflection && $parameter->getType() instanceof UnionType) {
+            $parameterType = $parameter->getType();
+            if (! $this->isValidUnionType($parameterType)) {
+                return null;
+            }
         }
 
         $args[$position]->value = new CastString_($argValue);