diff --git a/src/ServerRequestFactory.php b/src/ServerRequestFactory.php
index 3b767222..3bc5ede5 100644
--- a/src/ServerRequestFactory.php
+++ b/src/ServerRequestFactory.php
@@ -211,7 +211,11 @@ public static function marshalHeaders(array $server)
 
             if ($value && strpos($key, 'HTTP_') === 0) {
                 $name = strtr(strtolower(substr($key, 5)), '_', '-');
-                $headers[$name] = $value;
+
+                if (! is_numeric($name)) {
+                  $headers[$name] = $value;
+                }
+
                 continue;
             }
 
diff --git a/test/ServerRequestFactoryTest.php b/test/ServerRequestFactoryTest.php
index 63f202e1..35ebf53b 100644
--- a/test/ServerRequestFactoryTest.php
+++ b/test/ServerRequestFactoryTest.php
@@ -62,6 +62,7 @@ public function testMarshalsExpectedHeadersFromServerArray()
             'HTTP_CONTENT_TYPE' => 'application/json',
             'HTTP_ACCEPT' => 'application/json',
             'HTTP_X_FOO_BAR' => 'FOOBAR',
+            'HTTP__1' => '-1',
             'CONTENT_MD5' => 'CONTENT-MD5',
             'CONTENT_LENGTH' => 'UNSPECIFIED',
         ];