diff --git a/rust/hg-cpython/src/discovery.rs b/rust/hg-cpython/src/discovery.rs
--- a/rust/hg-cpython/src/discovery.rs
+++ b/rust/hg-cpython/src/discovery.rs
@@ -73,10 +73,14 @@
             }
         }
         let mut inner = self.inner(py).borrow_mut();
-        inner.add_common_revisions(common)
-            .map_err(|e| GraphError::pynew(py, e))?;
-        inner.add_missing_revisions(missing)
-            .map_err(|e| GraphError::pynew(py, e))?;
+        if !common.is_empty() {
+            inner.add_common_revisions(common)
+                .map_err(|e| GraphError::pynew(py, e))?;
+        }
+        if !missing.is_empty() {
+            inner.add_missing_revisions(missing)
+                .map_err(|e| GraphError::pynew(py, e))?;
+        }
         Ok(py.None())
     }