diff --git a/src/commands.cc b/src/commands.cc
index de7d0c98..8e58a7e6 100644
--- a/src/commands.cc
+++ b/src/commands.cc
@@ -842,6 +842,9 @@ void define_command(const ParametersParser& parser, Context& context, const Shel
     const String& cmd_name = parser[0];
     auto& cm = CommandManager::instance();
 
+    if (contains_that(cmd_name, is_blank))
+        throw runtime_error(format("invalid command name: '{}'", cmd_name));
+
     if (cm.command_defined(cmd_name) and not parser.get_switch("allow-override"))
         throw runtime_error(format("command '{}' already defined", cmd_name));