diff --git a/command.go b/command.go
index 0f4511f..3349be6 100644
--- a/command.go
+++ b/command.go
@@ -1012,7 +1012,7 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 
 func (c *Command) ValidateArgs(args []string) error {
 	if c.Args == nil {
-		return nil
+		return ArbitraryArgs(c, args)
 	}
 	return c.Args(c, args)
 }
diff --git a/user_guide.md b/user_guide.md
index 3a253de..5a7acf8 100644
--- a/user_guide.md
+++ b/user_guide.md
@@ -326,8 +326,8 @@ In both of these cases:
 
 ## Positional and Custom Arguments
 
-Validation of positional arguments can be specified using the `Args` field
-of `Command`.
+Validation of positional arguments can be specified using the `Args` field of `Command`.
+If `Args` is undefined or `nil`, it defaults to `ArbitraryArgs`.
 
 The following validators are built in: