diff --git a/control/argument.cpp b/control/argument.cpp
index 2852dee..d9b832d 100644
--- a/control/argument.cpp
+++ b/control/argument.cpp
@@ -82,6 +82,7 @@
 
 const char* ArgumentParser::optionstr = "ich?";
 
+const std::string ArgumentParser::true_string = "true";
 const std::string ArgumentParser::empty_string = "";
 
 }