diff --git a/src/Http/Request.php b/src/Http/Request.php
index 8332d99..c976212 100755
--- a/src/Http/Request.php
+++ b/src/Http/Request.php
@@ -332,7 +332,16 @@ abstract public function removeHeader(string $key): static;
      */
     public function getSize(): int
     {
-        return \mb_strlen(\implode("\n", $this->generateHeaders()), '8bit') + \mb_strlen(\file_get_contents('php://input'), '8bit');
+        $headers = $this->generateHeaders();
+        $headerStrings = [];
+        foreach ($headers as $key => $value) {
+            if (\is_array($value)) {
+                $headerStrings[] = $key . ': ' . \implode(', ', $value);
+            } else {
+                $headerStrings[] = $key . ': ' . $value;
+            }
+        }
+        return \mb_strlen(\implode("\n", $headerStrings), '8bit') + \mb_strlen(\file_get_contents('php://input'), '8bit');
     }
 
     /**
diff --git a/tests/RequestTest.php b/tests/RequestTest.php
index 55d49ee..02d8930 100755
--- a/tests/RequestTest.php
+++ b/tests/RequestTest.php
@@ -313,4 +313,23 @@ public function testCanGetRange()
         $this->assertEquals(null, $this->request->getRangeStart());
         $this->assertEquals(null, $this->request->getRangeEnd());
     }
+
+    public function testCanGetSizeWithArrayHeaders()
+    {
+        $this->request->addHeader('content-type', 'application/json');
+
+        $reflection = new \ReflectionClass($this->request);
+        $headersProperty = $reflection->getProperty('headers');
+        $headersProperty->setAccessible(true);
+
+        $headers = $headersProperty->getValue($this->request) ?? [];
+        $headers['accept'] = ['application/json', 'text/html'];
+        $headers['x-custom'] = ['value1', 'value2', 'value3'];
+        $headersProperty->setValue($this->request, $headers);
+
+        $size = $this->request->getSize();
+
+        $this->assertIsInt($size);
+        $this->assertGreaterThan(0, $size);
+    }
 }