diff --git a/rust/rhg/src/main.rs b/rust/rhg/src/main.rs
--- a/rust/rhg/src/main.rs
+++ b/rust/rhg/src/main.rs
@@ -69,6 +69,14 @@
 
     let (subcommand_name, subcommand_matches) = matches.subcommand();
 
+    // Mercurial allows users to define "defaults" for commands, fallback
+    // if a default is detected for the current command
+    let defaults = config.get_str(b"defaults", subcommand_name.as_bytes());
+    if defaults?.is_some() {
+        let msg = "`defaults` config set";
+        return Err(CommandError::unsupported(msg));
+    }
+
     for prefix in ["pre", "post", "fail"].iter() {
         // Mercurial allows users to define generic hooks for commands,
         // fallback if any are detected
diff --git a/tests/test-rhg.t b/tests/test-rhg.t
--- a/tests/test-rhg.t
+++ b/tests/test-rhg.t
@@ -228,6 +228,12 @@
   unsupported feature: fail-cat hook defined
   [252]
 
+Fallback with [defaults]
+  $ $NO_FALLBACK rhg cat original --config "defaults.cat=-r null"
+  unsupported feature: `defaults` config set
+  [252]
+
+
 Requirements
   $ $NO_FALLBACK rhg debugrequirements
   dotencode