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
@@ -247,6 +247,7 @@
     'ArcanistUnitRenderer' => 'unit/renderer/ArcanistUnitRenderer.php',
     'ArcanistUnitTestEngine' => 'unit/engine/ArcanistUnitTestEngine.php',
     'ArcanistUnitTestResult' => 'unit/ArcanistUnitTestResult.php',
+    'ArcanistUnitTestResultTestCase' => 'unit/__tests__/ArcanistUnitTestResultTestCase.php',
     'ArcanistUnitTestableLintEngine' => 'lint/engine/ArcanistUnitTestableLintEngine.php',
     'ArcanistUnitWorkflow' => 'workflow/ArcanistUnitWorkflow.php',
     'ArcanistUnnecessaryFinalModifierXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistUnnecessaryFinalModifierXHPASTLinterRule.php',
@@ -525,6 +526,7 @@
     'ArcanistUnitRenderer' => 'Phobject',
     'ArcanistUnitTestEngine' => 'Phobject',
     'ArcanistUnitTestResult' => 'Phobject',
+    'ArcanistUnitTestResultTestCase' => 'PhutilTestCase',
     'ArcanistUnitTestableLintEngine' => 'ArcanistLintEngine',
     'ArcanistUnitWorkflow' => 'ArcanistWorkflow',
     'ArcanistUnnecessaryFinalModifierXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
diff --git a/src/unit/ArcanistUnitTestResult.php b/src/unit/ArcanistUnitTestResult.php
--- a/src/unit/ArcanistUnitTestResult.php
+++ b/src/unit/ArcanistUnitTestResult.php
@@ -129,13 +129,22 @@
 
     $base = reset($coverage);
     foreach ($coverage as $more_coverage) {
-      $len = min(strlen($base), strlen($more_coverage));
+      $base_len = strlen($base);
+      $more_len = strlen($more_coverage);
+
+      $len = min($base_len, $more_len);
       for ($ii = 0; $ii < $len; $ii++) {
         if ($more_coverage[$ii] == 'C') {
           $base[$ii] = 'C';
         }
       }
+
+      // If a secondary report has more data, copy all of it over.
+      if ($more_len > $base_len) {
+        $base .= substr($more_coverage, $base_len);
+      }
     }
+
     return $base;
   }
 
diff --git a/src/unit/__tests__/ArcanistUnitTestResultTestCase.php b/src/unit/__tests__/ArcanistUnitTestResultTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/unit/__tests__/ArcanistUnitTestResultTestCase.php
@@ -0,0 +1,43 @@
+<?php
+
+final class ArcanistUnitTestResultTestCase extends PhutilTestCase {
+
+  public function testCoverageMerges() {
+    $cases = array(
+      array(
+        'coverage' => array(),
+        'expect' => null,
+      ),
+      array(
+        'coverage' => array(
+          'UUUNCNC',
+        ),
+        'expect' => 'UUUNCNC',
+      ),
+      array(
+        'coverage' => array(
+          'UUCUUU',
+          'UUUUCU',
+        ),
+        'expect' => 'UUCUCU',
+      ),
+      array(
+        'coverage' => array(
+          'UUCCCU',
+          'UUUCCCNNNC',
+        ),
+        'expect' => 'UUCCCCNNNC',
+      ),
+    );
+
+    foreach ($cases as $case) {
+      $input = $case['coverage'];
+      $expect = $case['expect'];
+
+      $actual = ArcanistUnitTestResult::mergeCoverage($input);
+
+      $this->assertEqual($expect, $actual);
+    }
+  }
+
+}