diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c
+++ b/mercurial/cext/revlog.c
@@ -2311,7 +2311,7 @@ rustlazyancestorsObject *rustlazyancesto
 	/* to pass index_get_parents() */
 	int (*)(indexObject *, Py_ssize_t, int*, int),
 	/* intrevs vector */
-	int initrevslen, long *initrevs,
+	Py_ssize_t initrevslen, long *initrevs,
 	long stoprev,
 	int inclusive);
 void rustlazyancestors_drop(rustlazyancestorsObject *self);
diff --git a/rust/hg-direct-ffi/src/ancestors.rs b/rust/hg-direct-ffi/src/ancestors.rs
--- a/rust/hg-direct-ffi/src/ancestors.rs
+++ b/rust/hg-direct-ffi/src/ancestors.rs
@@ -60,15 +60,16 @@ impl Graph for Index {
 pub extern "C" fn rustlazyancestors_init(
     index: IndexPtr,
     parents: IndexParentsFn,
-    initrevslen: usize,
+    initrevslen: ssize_t,
     initrevs: *mut c_long,
     stoprev: c_long,
     inclusive: c_int,
 ) -> *mut AncestorsIterator<Index> {
+    assert!(initrevslen >= 0);
     unsafe {
         raw_init(
             Index::new(index, parents),
-            initrevslen,
+            initrevslen as usize,
             initrevs,
             stoprev,
             inclusive,