diff --git a/src/util/system.h b/src/util/system.h
--- a/src/util/system.h
+++ b/src/util/system.h
@@ -285,12 +285,12 @@
     /**
      * Get the help string
      */
-    std::string GetHelpMessage();
+    std::string GetHelpMessage() const;
 
     /**
      * Check whether we know of this arg
      */
-    bool IsArgKnown(const std::string &key, std::string &error);
+    bool IsArgKnown(const std::string &key) const;
 };
 
 extern ArgsManager gArgs;
diff --git a/src/util/system.cpp b/src/util/system.cpp
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -483,7 +483,7 @@
 
         // Check that the arg is known
         if (!(IsSwitchChar(key[0]) && key.size() == 1)) {
-            if (!IsArgKnown(key, error)) {
+            if (!IsArgKnown(key)) {
                 error = strprintf("Invalid parameter %s", key.c_str());
                 return false;
             }
@@ -506,7 +506,7 @@
     return true;
 }
 
-bool ArgsManager::IsArgKnown(const std::string &key, std::string &error) {
+bool ArgsManager::IsArgKnown(const std::string &key) const {
     size_t option_index = key.find('.');
     std::string arg_no_net;
     if (option_index == std::string::npos) {
@@ -673,7 +673,7 @@
     m_config_args.erase(strArg);
 }
 
-std::string ArgsManager::GetHelpMessage() {
+std::string ArgsManager::GetHelpMessage() const {
     const bool show_debug = gArgs.GetBoolArg("-help-debug", false);
 
     std::string usage = "";
@@ -950,7 +950,7 @@
         }
 
         // Check that the arg is known
-        if (!IsArgKnown(strKey, error) && !ignore_invalid_keys) {
+        if (!IsArgKnown(strKey) && !ignore_invalid_keys) {
             error = strprintf("Invalid configuration value %s",
                               option.first.c_str());
             return false;