diff --git a/builtin/replay.c b/builtin/replay.c
index 69c4c55129..524bf96ffd 100644
--- a/builtin/replay.c
+++ b/builtin/replay.c
@@ -162,12 +162,12 @@ static void get_ref_information(struct repository *repo,
 	}
 }
 
-static void determine_replay_mode(struct repository *repo,
-				  struct rev_cmdline_info *cmd_info,
-				  const char *onto_name,
-				  char **advance_name,
-				  struct commit **onto,
-				  struct strset **update_refs)
+static void set_up_replay_mode(struct repository *repo,
+			       struct rev_cmdline_info *cmd_info,
+			       const char *onto_name,
+			       char **advance_name,
+			       struct commit **onto,
+			       struct strset **update_refs)
 {
 	struct ref_info rinfo;
 
@@ -182,10 +182,16 @@ static void determine_replay_mode(struct repository *repo,
 		if (rinfo.positive_refexprs <
 		    strset_get_size(&rinfo.positive_refs))
 			die(_("all positive revisions given must be references"));
-	} else if (*advance_name) {
+		*update_refs = xcalloc(1, sizeof(**update_refs));
+		**update_refs = rinfo.positive_refs;
+		memset(&rinfo.positive_refs, 0, sizeof(**update_refs));
+	} else {
 		struct object_id oid;
 		char *fullname = NULL;
 
+		if (!*advance_name)
+			BUG("expected either onto_name or *advance_name in this function");
+
 		*onto = peel_committish(repo, *advance_name);
 		if (repo_dwim_ref(repo, *advance_name, strlen(*advance_name),
 			     &oid, &fullname, 0) == 1) {
@@ -196,51 +202,6 @@ static void determine_replay_mode(struct repository *repo,
 		}
 		if (rinfo.positive_refexprs > 1)
 			die(_("cannot advance target with multiple sources because ordering would be ill-defined"));
-	} else {
-		int positive_refs_complete = (
-			rinfo.positive_refexprs ==
-			strset_get_size(&rinfo.positive_refs));
-		int negative_refs_complete = (
-			rinfo.negative_refexprs ==
-			strset_get_size(&rinfo.negative_refs));
-		/*
-		 * We need either positive_refs_complete or
-		 * negative_refs_complete, but not both.
-		 */
-		if (rinfo.negative_refexprs > 0 &&
-		    positive_refs_complete == negative_refs_complete)
-			die(_("cannot implicitly determine whether this is an --advance or --onto operation"));
-		if (negative_refs_complete) {
-			struct hashmap_iter iter;
-			struct strmap_entry *entry;
-			const char *last_key = NULL;
-
-			if (rinfo.negative_refexprs == 0)
-				die(_("all positive revisions given must be references"));
-			else if (rinfo.negative_refexprs > 1)
-				die(_("cannot implicitly determine whether this is an --advance or --onto operation"));
-			else if (rinfo.positive_refexprs > 1)
-				die(_("cannot advance target with multiple source branches because ordering would be ill-defined"));
-
-			/* Only one entry, but we have to loop to get it */
-			strset_for_each_entry(&rinfo.negative_refs,
-					      &iter, entry) {
-				last_key = entry->key;
-			}
-
-			free(*advance_name);
-			*advance_name = xstrdup_or_null(last_key);
-		} else { /* positive_refs_complete */
-			if (rinfo.negative_refexprs > 1)
-				die(_("cannot implicitly determine correct base for --onto"));
-			if (rinfo.negative_refexprs == 1)
-				*onto = rinfo.onto;
-		}
-	}
-	if (!*advance_name) {
-		*update_refs = xcalloc(1, sizeof(**update_refs));
-		**update_refs = rinfo.positive_refs;
-		memset(&rinfo.positive_refs, 0, sizeof(**update_refs));
 	}
 	strset_clear(&rinfo.negative_refs);
 	strset_clear(&rinfo.positive_refs);
@@ -451,8 +412,9 @@ int cmd_replay(int argc,
 		revs.simplify_history = 0;
 	}
 
-	determine_replay_mode(repo, &revs.cmdline, onto_name, &advance_name,
-			      &onto, &update_refs);
+	set_up_replay_mode(repo, &revs.cmdline,
+			   onto_name, &advance_name,
+			   &onto, &update_refs);
 
 	if (!onto) /* FIXME: Should handle replaying down to root commit */
 		die("Replaying down to root commit is not supported yet!");