diff --git a/CHANGELOG.md b/CHANGELOG.md
index 71762ca..d5d75b1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Unreleased
 
+Backwards-incompatible changes:
+
+* Mark `Command` and `Argument` as `non_exhaustive`, to allow extending them
+  in the future.
+
 Bug fixes:
 
 * Relax dependency version requirements (was tilde requirements).
\ No newline at end of file
diff --git a/src/command.rs b/src/command.rs
index a525478..17f495d 100644
--- a/src/command.rs
+++ b/src/command.rs
@@ -1,5 +1,6 @@
 /// A block, consisting of multiple commands.
 #[derive(Debug, PartialEq)]
+#[non_exhaustive]
 pub(crate) struct Block {
     /// The commands in the block.
     pub commands: Vec<Command>,
@@ -9,6 +10,7 @@ pub(crate) struct Block {
 
 /// A command.
 #[derive(Debug, PartialEq)]
+#[non_exhaustive]
 pub struct Command {
     /// The name of the command. Never empty.
     pub name: String,
@@ -23,6 +25,7 @@ pub struct Command {
 
 /// A command argument.
 #[derive(Debug, PartialEq)]
+#[non_exhaustive]
 pub struct Argument {
     /// The argument key, for `key=value` style arguments. Not guaranteed to be
     /// unique, the [`Runner`](crate::Runner) can handle this as desired.