diff --git a/impression/class_impression.py b/impression/class_impression.py
index e759c837..ec495e17 100755
--- a/impression/class_impression.py
+++ b/impression/class_impression.py
@@ -157,7 +157,7 @@ class fichier_impression :
                     options += ' -o sides=two-sided-short-edge'
                 else:
                     options += ' -o sides=one-sided'
-            options += ' -o HPColorasGray=%s' + (not self.couleur)
+            options += ' -o HPColorasGray=%s' % (not self.couleur)
             (status,rep) = commands.getstatusoutput("lpr %s %s" % (options,
                                                                    self.nom_fichier_pdf))
             if status != 0: