diff --git a/src/applications/diffusion/protocol/DiffusionSubversionWireProtocol.php b/src/applications/diffusion/protocol/DiffusionSubversionWireProtocol.php
index 934969f792..e5645859a4 100644
--- a/src/applications/diffusion/protocol/DiffusionSubversionWireProtocol.php
+++ b/src/applications/diffusion/protocol/DiffusionSubversionWireProtocol.php
@@ -75,6 +75,10 @@ final class DiffusionSubversionWireProtocol extends Phobject {
         }
       } else if ($this->state == 'bytes') {
         $new_data = substr($this->buffer, 0, $this->expectBytes);
+        if (!strlen($new_data)) {
+          // No more bytes available yet, wait for more data.
+          break;
+        }
         $this->buffer = substr($this->buffer, strlen($new_data));
 
         $this->expectBytes -= strlen($new_data);
diff --git a/src/applications/diffusion/protocol/__tests__/DiffusionSubversionWireProtocolTestCase.php b/src/applications/diffusion/protocol/__tests__/DiffusionSubversionWireProtocolTestCase.php
index 28b6473eb8..088f7222ed 100644
--- a/src/applications/diffusion/protocol/__tests__/DiffusionSubversionWireProtocolTestCase.php
+++ b/src/applications/diffusion/protocol/__tests__/DiffusionSubversionWireProtocolTestCase.php
@@ -61,6 +61,31 @@ final class DiffusionSubversionWireProtocolTestCase
       ));
   }
 
+  public function testSubversionWireProtocolPartialFrame() {
+    $proto = new DiffusionSubversionWireProtocol();
+
+    // This is primarily a test that we don't hang when we write() a frame
+    // which straddles a string boundary.
+    $msg1 = $proto->writeData('( duck 5:qu');
+    $msg2 = $proto->writeData('ack ) ');
+
+    $this->assertEqual(array(), ipull($msg1, 'structure'));
+    $this->assertEqual(
+      array(
+        array(
+          array(
+            'type' => 'word',
+            'value' => 'duck',
+          ),
+          array(
+            'type'=> 'string',
+            'value' => 'quack',
+          ),
+        ),
+      ),
+      ipull($msg2, 'structure'));
+  }
+
   private function assertSameSubversionMessages($string, array $structs) {
     $proto = new DiffusionSubversionWireProtocol();