diff --git a/rules-tests/CodeQuality/Rector/MethodCall/AssertIssetToSpecificMethodRector/Fixture/different_case_method.php.inc b/rules-tests/CodeQuality/Rector/MethodCall/AssertIssetToSpecificMethodRector/Fixture/different_case_method.php.inc
new file mode 100644
index 00000000..9f8cb599
--- /dev/null
+++ b/rules-tests/CodeQuality/Rector/MethodCall/AssertIssetToSpecificMethodRector/Fixture/different_case_method.php.inc
@@ -0,0 +1,29 @@
+<?php
+
+namespace Rector\PHPUnit\Tests\CodeQuality\Rector\MethodCall\AssertIssetToSpecificMethodRector\Fixture;
+
+final class DifferentCaseMethodTest extends \PHPUnit\Framework\TestCase
+{
+    public function test()
+    {
+        $this->asserttrue(isset($node['value1']), 'message');
+        $this->ASSERTFALSE(isset($node['value2']));
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\PHPUnit\Tests\CodeQuality\Rector\MethodCall\AssertIssetToSpecificMethodRector\Fixture;
+
+final class DifferentCaseMethodTest extends \PHPUnit\Framework\TestCase
+{
+    public function test()
+    {
+        $this->assertArrayHasKey('value1', $node, 'message');
+        $this->assertArrayNotHasKey('value2', $node);
+    }
+}
+
+?>
diff --git a/src/NodeAnalyzer/IdentifierManipulator.php b/src/NodeAnalyzer/IdentifierManipulator.php
index 636e872e..6b368a45 100644
--- a/src/NodeAnalyzer/IdentifierManipulator.php
+++ b/src/NodeAnalyzer/IdentifierManipulator.php
@@ -37,8 +37,15 @@ public function renameNodeWithMap(
             return false;
         }
 
-        $node->name = new Identifier($renameMethodMap[$oldNodeMethodName]);
-        return true;
+        // method names are case-insensitive in PHP, match the map the same way
+        foreach ($renameMethodMap as $oldName => $newName) {
+            if (strcasecmp($oldName, $oldNodeMethodName) === 0) {
+                $node->name = new Identifier($newName);
+                return true;
+            }
+        }
+
+        return false;
     }
 
     private function resolveOldMethodName(