diff --git a/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var.php.inc b/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var.php.inc
new file mode 100644
index 000000000000..a18807db65e3
--- /dev/null
+++ b/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var.php.inc
@@ -0,0 +1,16 @@
+<?php
+
+namespace Rector\Tests\Php71\Rector\Assign\AssignArrayToStringRector\Fixture;
+
+class SkipCastUndefinedVar
+{
+    public function fun()
+    {
+        $array[] = 'foo';
+        $array[] = 'bar';
+
+        return $array;
+    }
+}
+
+?>
diff --git a/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var2.php.inc b/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var2.php.inc
new file mode 100644
index 000000000000..c033b5809bea
--- /dev/null
+++ b/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var2.php.inc
@@ -0,0 +1,18 @@
+<?php
+
+namespace Rector\Tests\Php71\Rector\Assign\AssignArrayToStringRector\Fixture;
+
+class SkipCastUndefinedVar2
+{
+    private $array;
+
+    public function fun()
+    {
+        $array[] = 'foo';
+        $array[] = 'bar';
+
+        return $array;
+    }
+}
+
+?>
diff --git a/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var3.php.inc b/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var3.php.inc
new file mode 100644
index 000000000000..2c76e80c6913
--- /dev/null
+++ b/rules-tests/Php71/Rector/Assign/AssignArrayToStringRector/Fixture/skip_cast_undefined_var3.php.inc
@@ -0,0 +1,20 @@
+<?php
+
+namespace Rector\Tests\Php71\Rector\Assign\AssignArrayToStringRector\Fixture;
+
+class SkipCastUndefinedVar3
+{
+    public function fun()
+    {
+        $array = [];
+
+        function () {
+            $array[] = 'foo';
+            $array[] = 'bar';
+
+            return $array;
+        };
+    }
+}
+
+?>
diff --git a/rules/Php71/Rector/Assign/AssignArrayToStringRector.php b/rules/Php71/Rector/Assign/AssignArrayToStringRector.php
index 3df18de98d2a..fd3dae644661 100644
--- a/rules/Php71/Rector/Assign/AssignArrayToStringRector.php
+++ b/rules/Php71/Rector/Assign/AssignArrayToStringRector.php
@@ -105,9 +105,17 @@ public function refactor(Node $node): ?Node
             return $node;
         }
 
+        $isFoundPrev = (bool) $this->betterNodeFinder->findFirstPreviousOfNode($variable, function (Node $node) use ($variable): bool {
+            return $this->nodeComparator->areNodesEqual($node, $variable);
+        });
+
+        if (! $isFoundPrev) {
+            return null;
+        }
+
         // there is "$string[] = ...;", which would cause error in PHP 7+
         // fallback - if no array init found, retype to (array)
-        $assign = new Assign($arrayDimFetchNode->var, new ArrayCast($arrayDimFetchNode->var));
+        $assign = new Assign($variable, new ArrayCast($variable));
         $this->addNodeAfterNode(clone $node, $node);
 
         return $assign;
diff --git a/src/PhpParser/Node/BetterNodeFinder.php b/src/PhpParser/Node/BetterNodeFinder.php
index 659656170f75..323e7763a5d6 100644
--- a/src/PhpParser/Node/BetterNodeFinder.php
+++ b/src/PhpParser/Node/BetterNodeFinder.php
@@ -8,6 +8,7 @@
 use PhpParser\Node\Expr;
 use PhpParser\Node\Expr\Assign;
 use PhpParser\Node\Expr\Variable;
+use PhpParser\Node\FunctionLike;
 use PhpParser\Node\Stmt;
 use PhpParser\Node\Stmt\Class_;
 use PhpParser\Node\Stmt\ClassLike;
@@ -280,6 +281,10 @@ public function findFirstPreviousOfNode(Node $node, callable $filter): ?Node
         }
 
         $parent = $node->getAttribute(AttributeKey::PARENT_NODE);
+        if ($parent instanceof FunctionLike) {
+            return null;
+        }
+
         if ($parent instanceof Node) {
             return $this->findFirstPreviousOfNode($parent, $filter);
         }