diff --git a/rust/hg-core/src/config/config_items.rs b/rust/hg-core/src/config/config_items.rs
--- a/rust/hg-core/src/config/config_items.rs
+++ b/rust/hg-core/src/config/config_items.rs
@@ -198,7 +198,7 @@ impl TryFrom<&DefaultConfigItem> for Opt
                     _ => Err(err),
                 }
             }
-            None => Ok(Some(false)),
+            None => Ok(None),
         }
     }
 }
diff --git a/rust/hg-core/src/config/mod.rs b/rust/hg-core/src/config/mod.rs
--- a/rust/hg-core/src/config/mod.rs
+++ b/rust/hg-core/src/config/mod.rs
@@ -805,7 +805,6 @@ mod tests {
         assert!(ret.is_ok(), "{:?}", ret);
 
         let ret = config.get_byte_size(b"ui", b"formatted");
-        // FIXME should be `is_none()`
-        assert!(ret.unwrap().is_some());
+        assert!(ret.unwrap().is_none());
     }
 }