diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -299,6 +299,7 @@
     'PhutilPregsprintfTestCase' => 'xsprintf/__tests__/PhutilPregsprintfTestCase.php',
     'PhutilProcessGroupDaemon' => 'daemon/torture/PhutilProcessGroupDaemon.php',
     'PhutilProseDiff' => 'utils/PhutilProseDiff.php',
+    'PhutilProseDiffTestCase' => 'utils/__tests__/PhutilProseDiffTestCase.php',
     'PhutilProseDifferenceEngine' => 'utils/PhutilProseDifferenceEngine.php',
     'PhutilProtocolChannel' => 'channel/PhutilProtocolChannel.php',
     'PhutilProxyException' => 'error/PhutilProxyException.php',
@@ -862,6 +863,7 @@
     'PhutilPregsprintfTestCase' => 'PhutilTestCase',
     'PhutilProcessGroupDaemon' => 'PhutilTortureTestDaemon',
     'PhutilProseDiff' => 'Phobject',
+    'PhutilProseDiffTestCase' => 'PhutilTestCase',
     'PhutilProseDifferenceEngine' => 'Phobject',
     'PhutilProtocolChannel' => 'PhutilChannelChannel',
     'PhutilProxyException' => 'Exception',
diff --git a/src/utils/PhutilProseDiff.php b/src/utils/PhutilProseDiff.php
--- a/src/utils/PhutilProseDiff.php
+++ b/src/utils/PhutilProseDiff.php
@@ -63,10 +63,12 @@
       $type = $part['type'];
 
       if ($last !== $type) {
-        $combined[] = array(
-          'type' => $last,
-          'text' => $last_text,
-        );
+        if ($last !== null) {
+          $combined[] = array(
+            'type' => $last,
+            'text' => $last_text,
+          );
+        }
         $last_text = null;
         $last = $type;
       }
diff --git a/src/utils/__tests__/PhutilProseDiffTestCase.php b/src/utils/__tests__/PhutilProseDiffTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/utils/__tests__/PhutilProseDiffTestCase.php
@@ -0,0 +1,48 @@
+<?php
+
+final class PhutilProseDiffTestCase extends PhutilTestCase {
+
+  public function testProseDiffsDistance() {
+    $this->assertProseParts(
+      '',
+      '',
+      array(),
+      pht('Empty'));
+
+    $this->assertProseParts(
+      "xxx\nyyy",
+      "xxx\nzzz\nyyy",
+      array(
+        "= xxx\n",
+        "+ zzz\n",
+        '= yyy',
+      ),
+      pht('Add Paragraph'));
+
+    $this->assertProseParts(
+      "xxx\nzzz\nyyy",
+      "xxx\nyyy",
+      array(
+        "= xxx\n",
+        "- zzz\n",
+        '= yyy',
+      ),
+      pht('Remove Paragraph'));
+  }
+
+  private function assertProseParts($old, $new, array $expect_parts, $label) {
+    $engine = new PhutilProseDifferenceEngine();
+    $diff = $engine->getDiff($old, $new);
+
+    $parts = $diff->getParts();
+
+    $actual = array();
+    foreach ($parts as $part) {
+      $actual[] = $part['type'].' '.$part['text'];
+    }
+
+    $this->assertEqual($expect_parts, $actual, $label);
+  }
+
+
+}