diff --git a/libraries/plugins/witness/witness.cpp b/libraries/plugins/witness/witness.cpp
index 36606bd050..209e3368a8 100644
--- a/libraries/plugins/witness/witness.cpp
+++ b/libraries/plugins/witness/witness.cpp
@@ -546,6 +546,7 @@ void witness_plugin::start_mining(
              }
           }
        } );
+       thread_num++;
     }
 }