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
@@ -12,7 +12,7 @@
   > }
 
 Unimplemented command
-  $ rhg unimplemented-command
+  $ rhg unimplemented-command --config rhg.on-unsupported=abort
   unsupported feature: error: Found argument 'unimplemented-command' which wasn't expected, or isn't valid in this context
   
   USAGE:
@@ -153,22 +153,21 @@
   original content
 
 Fallback to Python
-  $ rhg cat original
+  $ rhg cat original --config rhg.on-unsupported=abort
   unsupported feature: `rhg cat` without `--rev` / `-r`
   [252]
-  $ FALLBACK="--config rhg.on-unsupported=fallback"
-  $ rhg cat original $FALLBACK
+  $ rhg cat original
   original content
 
-  $ rhg cat original $FALLBACK --config rhg.fallback-executable=false
+  $ rhg cat original --config rhg.fallback-executable=false
   [1]
 
-  $ rhg cat original $FALLBACK --config rhg.fallback-executable=hg-non-existent
+  $ rhg cat original --config rhg.fallback-executable=hg-non-existent
   tried to fall back to a 'hg-non-existent' sub-process but got error $ENOENT$
   unsupported feature: `rhg cat` without `--rev` / `-r`
   [252]
 
-  $ rhg cat original $FALLBACK --config rhg.fallback-executable="$RHG"
+  $ rhg cat original --config rhg.fallback-executable="$RHG"
   Blocking recursive fallback. The 'rhg.fallback-executable = */rust/target/release/rhg' config points to `rhg` itself. (glob)
   unsupported feature: `rhg cat` without `--rev` / `-r`
   [252]
@@ -183,15 +182,15 @@
   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]