diff --git a/Slim/App.php b/Slim/App.php
index 4abd33368..aff622e61 100644
--- a/Slim/App.php
+++ b/Slim/App.php
@@ -385,16 +385,25 @@ public function respond(ResponseInterface $response)
             if (!$contentLength) {
                 $contentLength = $body->getSize();
             }
-            $totalChunks    = ceil($contentLength / $chunkSize);
-            $lastChunkSize  = $contentLength % $chunkSize;
-            $currentChunk   = 0;
-            while (!$body->eof() && $currentChunk < $totalChunks) {
-                if (++$currentChunk == $totalChunks && $lastChunkSize > 0) {
-                    $chunkSize = $lastChunkSize;
+            if (isset($contentLength)) {
+                $totalChunks    = ceil($contentLength / $chunkSize);
+                $lastChunkSize  = $contentLength % $chunkSize;
+                $currentChunk   = 0;
+                while (!$body->eof() && $currentChunk < $totalChunks) {
+                    if (++$currentChunk == $totalChunks && $lastChunkSize > 0) {
+                        $chunkSize = $lastChunkSize;
+                    }
+                    echo $body->read($chunkSize);
+                    if (connection_status() != CONNECTION_NORMAL) {
+                        break;
+                    }
                 }
-                echo $body->read($chunkSize);
-                if (connection_status() != CONNECTION_NORMAL) {
-                    break;
+            } else {
+                while (!$body->eof()) {
+                    echo $body->read($chunkSize);
+                    if (connection_status() != CONNECTION_NORMAL) {
+                        break;
+                    }
                 }
             }
         }
diff --git a/tests/AppTest.php b/tests/AppTest.php
index 883d680a0..eeeae820e 100644
--- a/tests/AppTest.php
+++ b/tests/AppTest.php
@@ -1562,6 +1562,23 @@ public function testRespondWithPaddedStreamFilterOutput()
         }
     }
 
+    public function testRespondIndeterminateLength()
+    {
+        $app = new App();
+        $body_stream = fopen('php://temp', 'r+');
+        $response = new Response();
+        $body = $this->getMockBuilder("\Slim\Http\Body")
+            ->setMethods(["getSize"])
+            ->setConstructorArgs([$body_stream])
+            ->getMock();
+        fwrite($body_stream, "Hello");
+        rewind($body_stream);
+        $body->method("getSize")->willReturn(null);
+        $response = $response->withBody($body);
+        $app->respond($response);
+        $this->expectOutputString("Hello");
+    }
+
     public function testExceptionErrorHandlerDoesNotDisplayErrorDetails()
     {
         $app = new App();