diff --git a/cups-pdf-postprocess.py b/cups-pdf-postprocess.py
index 86646a6dc5658e7cbb0ed9365a416a52669a7eb3..99a35d2513a7cef598d5d4c179663945bce3749b 100755
--- a/cups-pdf-postprocess.py
+++ b/cups-pdf-postprocess.py
@@ -151,7 +151,7 @@ class CPP(QWidget):
     else:
       cmd = self._printer.currentData().copy()
       cmd += [ "-o", self._duplex.currentData() ]
-      if self._stample.isChecked:
+      if self._stample.isChecked():
         cmd += [ "-o", "XRFold=BiFoldStaple" ]
       cmd.append("-")
       pipeline.append(cmd)