diff --git a/src/Prefixer.php b/src/Prefixer.php
index a03a95b3..30ed6346 100644
--- a/src/Prefixer.php
+++ b/src/Prefixer.php
@@ -189,6 +189,7 @@ public function replaceNamespace(string $contents, string $originalNamespace, st
             |implements\s+
             |extends\s+                    # when the class being extended is namespaced inline
             |return\s+
+            |instanceof\s+                 # when checking the class type of an object in a conditional
             |\(\s*                         # inside a function declaration as the first parameters type
             |,\s*                          # inside a function declaration as a subsequent parameter type
             |\.\s*                         # as part of a concatenated string
diff --git a/tests/Issues/StraussIssue83Test.php b/tests/Issues/StraussIssue83Test.php
new file mode 100644
index 00000000..c56a9c37
--- /dev/null
+++ b/tests/Issues/StraussIssue83Test.php
@@ -0,0 +1,63 @@
+<?php
+/**
+ * instanceof not prefixed properly.
+ *
+ * @see https://github.com/BrianHenryIE/strauss/issues/83
+ */
+
+namespace BrianHenryIE\Strauss\Tests\Issues;
+
+use BrianHenryIE\Strauss\Console\Commands\Compose;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+
+/**
+ * @package BrianHenryIE\Strauss\Tests\Issues
+ * @coversNothing
+ */
+class StraussIssue83Test extends \BrianHenryIE\Strauss\Tests\Integration\Util\IntegrationTestCase
+{
+    public function test_namespace_keyword_on_opening_line()
+    {
+        self::markTestSkipped('Slow test.');
+
+        $composerJsonString = <<<'EOD'
+{
+  "name": "issue/83",
+  "require": {
+    "aws/aws-sdk-php": "3.293.8"
+  },
+  "extra": {
+    "strauss": {
+      "namespace_prefix": "Company\\Project\\",
+      "exclude_from_copy": {
+		  "file_patterns": [
+		    "/^((?!aws\\/aws-sdk-php).)*$/"
+		  ]
+      }
+    }
+  }
+}
+EOD;
+
+        chdir($this->testsWorkingDir);
+
+        file_put_contents($this->testsWorkingDir . '/composer.json', $composerJsonString);
+
+        exec('composer install');
+
+        $inputInterfaceMock = $this->createMock(InputInterface::class);
+        $outputInterfaceMock = $this->createMock(OutputInterface::class);
+
+        $strauss = new Compose();
+
+        $result = $strauss->run($inputInterfaceMock, $outputInterfaceMock);
+
+        self::assertEquals(0, $result);
+
+        $php_string = file_get_contents($this->testsWorkingDir . '/vendor-prefixed/aws/aws-sdk-php/src/ClientResolver.php');
+
+        self::assertStringNotContainsString('$value instanceof \Aws\EndpointV2\EndpointProviderV2', $php_string);
+        self::assertStringContainsString('$value instanceof \Company\Project\Aws\EndpointV2\EndpointProviderV2', $php_string);
+    }
+}
diff --git a/tests/Unit/PrefixerTest.php b/tests/Unit/PrefixerTest.php
index 695837ea..7d9c34ca 100644
--- a/tests/Unit/PrefixerTest.php
+++ b/tests/Unit/PrefixerTest.php
@@ -14,6 +14,7 @@
 use Composer\Composer;
 use Composer\Config;
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\Console\Input\InputInterface;
 
 /**
  * Class ReplacerTest
@@ -43,7 +44,9 @@ protected function aaasetUp(): void
         $composer = new Composer();
         $composer->setConfig($composerConfig);
 
-        $this->config = new StraussConfig($composer);
+        $input = $this->createMock(InputInterface::class);
+
+        $this->config = new StraussConfig($composer, $input);
     }
 
     public function testNamespaceReplacer()
@@ -1784,4 +1787,52 @@ public function get_google_client() {
 
         $this->assertEquals($expected, $result);
     }
+
+    /**
+     * @see https://github.com/BrianHenryIE/strauss/issues/83
+     * @see vendor-prefixed/aws/aws-sdk-php/src/ClientResolver.php:955
+     */
+    public function testPrefixesFullNamespaceInInstanceOf(): void
+    {
+        $contents = <<<'EOD'
+<?php
+namespace Aws;
+
+class ClientResolver
+	public static function _apply_user_agent($inputUserAgent, array &$args, HandlerList $list)
+    {
+            if (($args['endpoint_discovery'] instanceof \Aws\EndpointDiscovery\Configuration
+                && $args['endpoint_discovery']->isEnabled())
+            ) {
+            
+            }
+	}
+}
+EOD;
+
+        $expected = <<<'EOD'
+<?php
+namespace Company\Project\Aws;
+
+class ClientResolver
+	public static function _apply_user_agent($inputUserAgent, array &$args, HandlerList $list)
+    {
+            if (($args['endpoint_discovery'] instanceof \Company\Project\Aws\EndpointDiscovery\Configuration
+                && $args['endpoint_discovery']->isEnabled())
+            ) {
+            
+            }
+	}
+}
+EOD;
+
+        $config = $this->createMock(StraussConfig::class);
+
+        $replacer = new Prefixer($config, __DIR__);
+
+        $result = $replacer->replaceNamespace($contents, 'Aws\\EndpointDiscovery', 'Company\\Project\\Aws\\EndpointDiscovery');
+        $result = $replacer->replaceNamespace($result, 'Aws', 'Company\\Project\\Aws');
+
+        $this->assertEquals($expected, $result);
+    }
 }