diff --git a/rust/rhg/src/commands/cat.rs b/rust/rhg/src/commands/cat.rs
--- a/rust/rhg/src/commands/cat.rs
+++ b/rust/rhg/src/commands/cat.rs
@@ -46,6 +46,11 @@
 
     let mut files = vec![];
     for file in file_args.iter() {
+        if file.starts_with("set:") {
+            let message = "fileset";
+            return Err(CommandError::unsupported(message));
+        }
+
         let normalized = cwd.join(&file);
         // TODO: actually normalize `..` path segments etc?
         let dotted = normalized.components().any(|c| c.as_os_str() == "..");
diff --git a/tests/test-rhg.t b/tests/test-rhg.t
--- a/tests/test-rhg.t
+++ b/tests/test-rhg.t
@@ -210,6 +210,11 @@
   unsupported feature: `..` or `.` path segment
   [252]
 
+Fallback with filesets
+  $ $NO_FALLBACK rhg cat "set:c or b"
+  unsupported feature: fileset
+  [252]
+
 Requirements
   $ $NO_FALLBACK rhg debugrequirements
   dotencode