diff --git a/src/xsprintf/PhutilQueryString.php b/src/xsprintf/PhutilQueryString.php
--- a/src/xsprintf/PhutilQueryString.php
+++ b/src/xsprintf/PhutilQueryString.php
@@ -2,16 +2,44 @@
 
 final class PhutilQueryString extends Phobject {
 
-  private $escaper;
-  private $argv;
+  private $maskedString;
+  private $unmaskedString;
 
   public function __construct(PhutilQsprintfInterface $escaper, array $argv) {
-    $this->escaper = $escaper;
-    $this->argv = $argv;
+    // Immediately render the query into a static scalar value.
 
     // This makes sure we throw immediately if there are errors in the
-    // parameters.
-    $this->getMaskedString();
+    // parameters, which is much better than throwing later on.
+
+    // This also makes sure that later mutations to objects passed as
+    // parameters won't affect the outcome. Consider:
+    //
+    //   $object->setTableName('X');
+    //   $query = qsprintf($conn, '%R', $object);
+    //   $object->setTableName('Y');
+    //
+    // We'd like "$query" to reference "X", reflecting the object as it
+    // existed when it was passed to "qsprintf(...)". It's surprising if the
+    // modification to the object after "qsprintf(...)" can affect "$query".
+
+    $masked_string = xsprintf(
+      'xsprintf_query',
+      array(
+        'escaper' => $escaper,
+        'unmasked' => false,
+      ),
+      $argv);
+
+    $unmasked_string = xsprintf(
+      'xsprintf_query',
+      array(
+        'escaper' => $escaper,
+        'unmasked' => true,
+      ),
+      $argv);
+
+    $this->maskedString = $masked_string;
+    $this->unmaskedString = $unmasked_string;
   }
 
   public function __toString() {
@@ -19,21 +47,11 @@
   }
 
   public function getUnmaskedString() {
-    return $this->renderString(true);
+    return $this->unmaskedString;
   }
 
   public function getMaskedString() {
-    return $this->renderString(false);
-  }
-
-  private function renderString($unmasked) {
-    return xsprintf(
-      'xsprintf_query',
-      array(
-        'escaper' => $this->escaper,
-        'unmasked' => $unmasked,
-      ),
-      $this->argv);
+    return $this->maskedString;
   }
 
 }