diff --git a/src/CachingStream.php b/src/CachingStream.php
index fe749e98..febade9f 100644
--- a/src/CachingStream.php
+++ b/src/CachingStream.php
@@ -36,7 +36,13 @@ public function __construct(
 
     public function getSize()
     {
-        return max($this->stream->getSize(), $this->remoteStream->getSize());
+        $remoteSize = $this->remoteStream->getSize();
+
+        if (null === $remoteSize) {
+            return null;
+        }
+
+        return max($this->stream->getSize(), $remoteSize);
     }
 
     public function rewind()
diff --git a/tests/CachingStreamTest.php b/tests/CachingStreamTest.php
index c760f5db..a1fa04ae 100644
--- a/tests/CachingStreamTest.php
+++ b/tests/CachingStreamTest.php
@@ -34,13 +34,22 @@ public function tearDownTest()
         $this->body->close();
     }
 
-    public function testUsesRemoteSizeIfPossible()
+    public function testUsesRemoteSizeIfAvailable()
     {
         $body = Psr7\Utils::streamFor('test');
         $caching = new CachingStream($body);
         self::assertSame(4, $caching->getSize());
     }
 
+    public function testUsesRemoteSizeIfNotAvailable()
+    {
+        $body = new Psr7\PumpStream(function () {
+            return 'a';
+        });
+        $caching = new CachingStream($body);
+        self::assertNull($caching->getSize());
+    }
+
     public function testReadsUntilCachedToByte()
     {
         $this->body->seek(5);