diff --git a/rules-tests/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector/Fixture/crypt_one_arg.php.inc b/rules-tests/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector/Fixture/crypt_one_arg.php.inc
new file mode 100644
index 00000000000..c3a5e1223d0
--- /dev/null
+++ b/rules-tests/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector/Fixture/crypt_one_arg.php.inc
@@ -0,0 +1,37 @@
+<?php
+
+namespace Rector\Tests\Php80\Rector\FuncCall\NullToStrictStringFuncCallArgRector\Fixture;
+
+/**
+ * Crypt requires 2 args since php 5.6, when it only fill 1st arg, it will only change 1st arg
+ *
+ * Note: Filling 2nd arg will require different Rule to match the salt algorithm.
+ */
+final class CryptOneArg
+{
+    public function run($subject)
+    {
+        crypt($subject);
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\Php80\Rector\FuncCall\NullToStrictStringFuncCallArgRector\Fixture;
+
+/**
+ * Crypt requires 2 args since php 5.6, when it only fill 1st arg, it will only change 1st arg
+ *
+ * Note: Filling 2nd arg will require different Rule to match the salt algorithm.
+ */
+final class CryptOneArg
+{
+    public function run($subject)
+    {
+        crypt((string) $subject);
+    }
+}
+
+?>
diff --git a/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php b/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php
index 23a2e1fcdc8..c6f3b2e623b 100644
--- a/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php
+++ b/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php
@@ -391,6 +391,10 @@ private function processNullToStrictStringOnNodePosition(
         array $args,
         int|string $position
     ): ?FuncCall {
+        if (! isset($args[$position])) {
+            return null;
+        }
+
         $argValue = $args[$position]->value;
 
         if ($argValue instanceof ConstFetch && $this->valueResolver->isNull($argValue)) {
@@ -405,11 +409,7 @@ private function processNullToStrictStringOnNodePosition(
             return null;
         }
 
-        if (! $type instanceof MixedType) {
-            return null;
-        }
-
-        if ($argValue instanceof Encapsed) {
+        if (! $type instanceof MixedType || $argValue instanceof Encapsed) {
             return null;
         }