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
@@ -311,27 +311,27 @@
 }
 
 impl OnUnsupported {
-    const DEFAULT: Self = OnUnsupported::Abort;
     const DEFAULT_FALLBACK_EXECUTABLE: &'static [u8] = b"hg";
 
     fn from_config(config: &Config) -> Self {
-        match config
+        let fallback = b"fallback";
+        match &*config
             .get(b"rhg", b"on-unsupported")
-            .map(|value| value.to_ascii_lowercase())
-            .as_deref()
+            .unwrap_or(fallback)
+            .to_ascii_lowercase()
         {
-            Some(b"abort") => OnUnsupported::Abort,
-            Some(b"abort-silent") => OnUnsupported::AbortSilent,
-            Some(b"fallback") => OnUnsupported::Fallback {
-                executable: config
-                    .get(b"rhg", b"fallback-executable")
-                    .unwrap_or(Self::DEFAULT_FALLBACK_EXECUTABLE)
-                    .to_owned(),
-            },
-            None => Self::DEFAULT,
-            Some(_) => {
-                // TODO: warn about unknown config value
-                Self::DEFAULT
+            b"abort" => OnUnsupported::Abort,
+            b"abort-silent" => OnUnsupported::AbortSilent,
+            value => {
+                if value != fallback {
+                    // TODO: warn about unknown config value
+                }
+                OnUnsupported::Fallback {
+                    executable: config
+                        .get(b"rhg", b"fallback-executable")
+                        .unwrap_or(Self::DEFAULT_FALLBACK_EXECUTABLE)
+                        .to_owned(),
+                }
             }
         }
     }
diff --git a/tests/test-rhg.t b/tests/test-rhg.t
--- a/tests/test-rhg.t
+++ b/tests/test-rhg.t
@@ -11,6 +11,9 @@
   > fi
   > }
 
+  $ echo "[rhg]" >> $HGRCPATH
+  $ echo "on-unsupported = abort" >> $HGRCPATH
+
 Unimplemented command
   $ rhg unimplemented-command
   unsupported feature: error: Found argument 'unimplemented-command' which wasn't expected, or isn't valid in this context
@@ -183,15 +186,15 @@
   store
 
   $ echo indoor-pool >> .hg/requires
-  $ rhg files
+  $ rhg files --config rhg.on-unsupported=abort
   unsupported feature: repository requires feature unknown to this Mercurial: indoor-pool
   [252]
 
-  $ rhg cat -r 1 copy_of_original
+  $ rhg cat -r 1 copy_of_original --config rhg.on-unsupported=abort
   unsupported feature: repository requires feature unknown to this Mercurial: indoor-pool
   [252]
 
-  $ rhg debugrequirements
+  $ rhg debugrequirements --config rhg.on-unsupported=abort
   unsupported feature: repository requires feature unknown to this Mercurial: indoor-pool
   [252]