diff --git a/src/Curl/Curl.php b/src/Curl/Curl.php
index 0f8f921..3676b9e 100644
--- a/src/Curl/Curl.php
+++ b/src/Curl/Curl.php
@@ -1936,11 +1936,14 @@ class Curl
      */
     private function buildCookies()
     {
-        // Avoid using http_build_query() as unnecessary encoding is performed.
-        // http_build_query($this->cookies, '', '; ');
-        $this->setOpt(CURLOPT_COOKIE, implode('; ', array_map(function ($k, $v) {
-            return $k . '=' . $v;
-        }, array_keys($this->cookies), array_values($this->cookies))));
+        // Avoid changing CURLOPT_COOKIE if there are no cookies set.
+        if (count($this->cookies)) {
+            // Avoid using http_build_query() as unnecessary encoding is performed.
+            // http_build_query($this->cookies, '', '; ');
+            $this->setOpt(CURLOPT_COOKIE, implode('; ', array_map(function ($k, $v) {
+                return $k . '=' . $v;
+            }, array_keys($this->cookies), array_values($this->cookies))));
+        }
     }
 
     /**
diff --git a/tests/PHPCurlClass/PHPMultiCurlClassTest.php b/tests/PHPCurlClass/PHPMultiCurlClassTest.php
index 11d66b9..9ec5cc7 100644
--- a/tests/PHPCurlClass/PHPMultiCurlClassTest.php
+++ b/tests/PHPCurlClass/PHPMultiCurlClassTest.php
@@ -2310,6 +2310,73 @@ class MultiCurlTest extends \PHPUnit\Framework\TestCase
         $this->assertEquals('yummy', $get_2->responseCookies['mycookie']);
     }
 
+    public function testSetCookieStringMultiCurl()
+    {
+        $cookie_string = 'fruit=apple; color=red';
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'cookie');
+        $multi_curl->setCookieString($cookie_string);
+
+        $get_1 = $multi_curl->addGet(Test::TEST_URL);
+        $get_1->complete(function ($instance) use ($cookie_string) {
+            \PHPUnit\Framework\Assert::assertEquals('fruit=apple&color=red', $instance->response);
+            \PHPUnit\Framework\Assert::assertEquals($cookie_string, $instance->getOpt(CURLOPT_COOKIE));
+        });
+
+        $multi_curl->start();
+    }
+
+    public function testSetCookieStringCurl()
+    {
+        $cookie_string = 'fruit=apple; color=red';
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'cookie');
+
+        $get_1 = $multi_curl->addGet(Test::TEST_URL);
+        $get_1->setCookieString($cookie_string);
+        $get_1->complete(function ($instance) use ($cookie_string) {
+            \PHPUnit\Framework\Assert::assertEquals('fruit=apple&color=red', $instance->response);
+            \PHPUnit\Framework\Assert::assertEquals($cookie_string, $instance->getOpt(CURLOPT_COOKIE));
+        });
+
+        $multi_curl->start();
+    }
+
+    public function testSetCookieString()
+    {
+        $cookie_string_1 = 'fruit=apple; color=red';
+        $cookie_string_2 = 'fruit=banana; color=yellow';
+        $cookie_string_3 = 'fruit=orange; color=orange';
+
+        $multi_curl = new MultiCurl();
+        $multi_curl->setHeader('X-DEBUG-TEST', 'cookie');
+        $multi_curl->setCookieString($cookie_string_1);
+
+        $get_1 = $multi_curl->addGet(Test::TEST_URL);
+        $get_1->complete(function ($instance) use ($cookie_string_1) {
+            \PHPUnit\Framework\Assert::assertEquals('fruit=apple&color=red', $instance->response);
+            \PHPUnit\Framework\Assert::assertEquals($cookie_string_1, $instance->getOpt(CURLOPT_COOKIE));
+        });
+
+        $get_2 = $multi_curl->addGet(Test::TEST_URL);
+        $get_2->setCookieString($cookie_string_2);
+        $get_2->complete(function ($instance) use ($cookie_string_2) {
+            \PHPUnit\Framework\Assert::assertEquals('fruit=banana&color=yellow', $instance->response);
+            \PHPUnit\Framework\Assert::assertEquals($cookie_string_2, $instance->getOpt(CURLOPT_COOKIE));
+        });
+
+        $get_3 = $multi_curl->addGet(Test::TEST_URL);
+        $get_3->setCookieString($cookie_string_3);
+        $get_3->complete(function ($instance) use ($cookie_string_3) {
+            \PHPUnit\Framework\Assert::assertEquals('fruit=orange&color=orange', $instance->response);
+            \PHPUnit\Framework\Assert::assertEquals($cookie_string_3, $instance->getOpt(CURLOPT_COOKIE));
+        });
+
+        $multi_curl->start();
+    }
+
     public function testJsonRequest()
     {
         foreach ([