diff --git a/src/Framework/Constraint/Count.php b/src/Framework/Constraint/Count.php
index 7aeb2d72fcc..6973d0d0eb4 100644
--- a/src/Framework/Constraint/Count.php
+++ b/src/Framework/Constraint/Count.php
@@ -22,6 +22,11 @@ class Count extends Constraint
      */
     private $expectedCount;
 
+    /**
+     * @var null|int
+     */
+    private $countOfGenerator;
+
     public function __construct(int $expected)
     {
         parent::__construct();
@@ -93,11 +98,13 @@ protected function getCountOf($other): ?int
      */
     protected function getCountOfGenerator(Generator $generator): int
     {
-        for ($count = 0; $generator->valid(); $generator->next()) {
-            ++$count;
+        if ($this->countOfGenerator === null) {
+            for ($this->countOfGenerator = 0; $generator->valid(); $generator->next()) {
+                ++$this->countOfGenerator;
+            }
         }
 
-        return $count;
+        return $this->countOfGenerator;
     }
 
     /**
diff --git a/tests/_files/GeneratorTest.php b/tests/_files/GeneratorTest.php
new file mode 100644
index 00000000000..12487f759dc
--- /dev/null
+++ b/tests/_files/GeneratorTest.php
@@ -0,0 +1,28 @@
+<?php
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+namespace Test;
+
+use PHPUnit\Framework\TestCase;
+
+class GeneratorTest extends TestCase
+{
+    public function testGenerator()
+    {
+        $generator = static function () {
+            yield 1;
+
+            yield 2;
+
+            yield 3;
+        };
+
+        static::assertCount(4, $generator());
+    }
+}
diff --git a/tests/end-to-end/generator-failing-assert-count.phpt b/tests/end-to-end/generator-failing-assert-count.phpt
new file mode 100644
index 00000000000..d51974c9712
--- /dev/null
+++ b/tests/end-to-end/generator-failing-assert-count.phpt
@@ -0,0 +1,26 @@
+--TEST--
+phpunit BankAccountTest ../../_files/GeneratorTest.php
+--FILE--
+<?php
+$_SERVER['argv'][1] = '--no-configuration';
+$_SERVER['argv'][3] = __DIR__ . '/../_files/GeneratorTest.php';
+
+require __DIR__ . '/../bootstrap.php';
+PHPUnit\TextUI\Command::main();
+?>
+--EXPECTF--
+PHPUnit %s by Sebastian Bergmann and contributors.
+
+F                                                                   1 / 1 (100%)
+
+Time: %s, Memory: %s
+
+There was 1 failure:
+
+1) Test\GeneratorTest::testGenerator
+Failed asserting that actual size 3 matches expected size 4.
+
+%sGeneratorTest.php:%i
+
+FAILURES!
+Tests: 1, Assertions: 1, Failures: 1.