diff --git a/rust/hg-core/src/requirements.rs b/rust/hg-core/src/requirements.rs
--- a/rust/hg-core/src/requirements.rs
+++ b/rust/hg-core/src/requirements.rs
@@ -24,7 +24,7 @@
         .map(|line| {
             // Python uses Unicode `str.isalnum` but feature names are all
             // ASCII
-            if line[0].is_ascii_alphanumeric() {
+            if line[0].is_ascii_alphanumeric() && line.is_ascii() {
                 Ok(String::from_utf8(line.into()).unwrap())
             } else {
                 Err(())
diff --git a/tests/test-rhg.t b/tests/test-rhg.t
--- a/tests/test-rhg.t
+++ b/tests/test-rhg.t
@@ -140,3 +140,8 @@
   sparserevlog
   store
   indoor-pool
+
+  $ echo -e '\xFF' >> .hg/requires
+  $ rhg debugrequirements
+  abort: .hg/requires is corrupted
+  [255]