diff --git a/src/detail/face-impl.hpp b/src/detail/face-impl.hpp
index 8e48750..cd1b3d8 100644
--- a/src/detail/face-impl.hpp
+++ b/src/detail/face-impl.hpp
@@ -270,8 +270,11 @@
         (*i)->unregister(bind(&Impl::finalizeUnregisterPrefix, this, i, onSuccess),
                          bind(onFailure, _2));
       }
-    else
-      onFailure("Unrecognized PrefixId");
+    else {
+      if (static_cast<bool>(onFailure)) {
+        onFailure("Unrecognized PrefixId");
+      }
+    }
 
     // there cannot be two registered prefixes with the same id
   }