diff --git a/src/main/scala/leon/synthesis/Synthesizer.scala b/src/main/scala/leon/synthesis/Synthesizer.scala
index 5b95fa9f5685f2fff24caa7873ef7682564f94a5..8e2953edf6c6c2d42f2c0610010cae2c745ff3ac 100644
--- a/src/main/scala/leon/synthesis/Synthesizer.scala
+++ b/src/main/scala/leon/synthesis/Synthesizer.scala
@@ -92,7 +92,7 @@ class Synthesizer(val reporter: Reporter,
           }
 
         case t: TaskRunRule=>
-          val prefix = "[%-20s] ".format(Option(t.rule))
+          val prefix = "[%-20s] ".format(Option(t.rule).getOrElse("?"))
 
           t.app.apply() match {
             case RuleSuccess(sol) =>