diff --git a/doc/release-notes.md b/doc/release-notes.md
--- a/doc/release-notes.md
+++ b/doc/release-notes.md
@@ -19,3 +19,10 @@
     and purpose key/value pairs has been deprecated in favor of an array of
     label names and will be removed in a future release. The previous behavior can be
     re-enabled in the interim by launching with `-deprecatedrpc=labelspurpose`.
+
+Command line
+------------
+
+Command line options prefixed with main/test/regtest network names like
+`-main.port=8333` `-test.server=1` previously were allowed but ignored. Now
+they trigger "Invalid parameter" errors on startup.
diff --git a/src/test/util_tests.cpp b/src/test/util_tests.cpp
--- a/src/test/util_tests.cpp
+++ b/src/test/util_tests.cpp
@@ -436,6 +436,26 @@
     }
 }
 
+BOOST_AUTO_TEST_CASE(util_ParseInvalidParameters) {
+    TestArgsManager test;
+    test.SetupArgs({{"-registered", ArgsManager::ALLOW_ANY}});
+
+    const char *argv[] = {"ignored", "-registered"};
+    std::string error;
+    BOOST_CHECK(test.ParseParameters(2, (char **)argv, error));
+    BOOST_CHECK_EQUAL(error, "");
+
+    argv[1] = "-unregistered";
+    BOOST_CHECK(!test.ParseParameters(2, (char **)argv, error));
+    BOOST_CHECK_EQUAL(error, "Invalid parameter -unregistered");
+
+    // Make sure registered parameters prefixed with a chain name trigger
+    // errors. (Previously, they were accepted and ignored.)
+    argv[1] = "-test.registered";
+    BOOST_CHECK(!test.ParseParameters(2, (char **)argv, error));
+    BOOST_CHECK_EQUAL(error, "Invalid parameter -test.registered");
+}
+
 static void TestParse(const std::string &str, bool expected_bool,
                       int64_t expected_int) {
     TestArgsManager test;
@@ -1002,7 +1022,9 @@
     //! Enumerate all possible test configurations.
     template <typename Fn> void ForEachMergeSetup(Fn &&fn) {
         ActionList arg_actions = {};
-        ForEachNoDup(arg_actions, SET, SECTION_NEGATE, [&] {
+        // command_line_options do not have sections. Only iterate over SET and
+        // NEGATE
+        ForEachNoDup(arg_actions, SET, NEGATE, [&] {
             ActionList conf_actions = {};
             ForEachNoDup(conf_actions, SET, SECTION_NEGATE, [&] {
                 for (bool soft_set : {false, true}) {
@@ -1189,7 +1211,7 @@
     //   <GetUnsuitable output>
     BOOST_CHECK_EQUAL(
         out_sha_hex,
-        "b835eef5977d69114eb039a976201f8c7121f34fe2b7ea2b73cafb516e5c9dc8");
+        "8fd4877bb8bf337badca950ede6c917441901962f160e52514e06a60dea46cde");
 }
 
 // Similar test as above, but for ArgsManager::GetChainName function.
diff --git a/src/util/system.cpp b/src/util/system.cpp
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -336,21 +336,20 @@
         std::string section;
         util::SettingsValue value = InterpretOption(section, key, val);
         std::optional<unsigned int> flags = GetArgFlags('-' + key);
-        if (flags) {
-            if (!CheckValid(key, value, *flags, error)) {
-                return false;
-            }
-            // Weird behavior preserved for backwards compatibility: command
-            // line options with section prefixes are allowed but ignored. It
-            // would be better if these options triggered the Invalid parameter
-            // error below.
-            if (section.empty()) {
-                m_settings.command_line_options[key].push_back(value);
-            }
-        } else {
-            error = strprintf("Invalid parameter -%s", key);
+
+        // Unknown command line options and command line options with dot
+        // characters (which are returned from InterpretOption with nonempty
+        // section strings) are not valid.
+        if (!flags || !section.empty()) {
+            error = strprintf("Invalid parameter %s", argv[i]);
             return false;
         }
+
+        if (!CheckValid(key, value, *flags, error)) {
+            return false;
+        }
+
+        m_settings.command_line_options[key].push_back(value);
     }
 
     // we do not allow -includeconf from command line
diff --git a/test/functional/feature_config_args.py b/test/functional/feature_config_args.py
--- a/test/functional/feature_config_args.py
+++ b/test/functional/feature_config_args.py
@@ -23,7 +23,7 @@
             conf.write('includeconf={}\n'.format(inc_conf_file_path))
 
         self.nodes[0].assert_start_raises_init_error(
-            expected_msg='Error: Error parsing command line arguments: Invalid parameter -dash_cli',
+            expected_msg='Error: Error parsing command line arguments: Invalid parameter -dash_cli=1',
             extra_args=['-dash_cli=1'],
         )
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf: