diff --git a/builtin/rebase.c b/builtin/rebase.c
index 74024f49df..9ee3ab6240 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -865,13 +865,9 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
 	struct commit_list *merge_bases = NULL;
 	int res = 0;
 
-	merge_bases = get_merge_bases(onto, head);
-	if (!merge_bases || merge_bases->next) {
-		oidcpy(branch_base, null_oid());
-		goto done;
-	}
+	if (is_null_oid(branch_base))
+		goto done; /* fill_branch_base() found multiple merge bases */
 
-	oidcpy(branch_base, &merge_bases->item->object.oid);
 	if (!oideq(branch_base, &onto->object.oid))
 		goto done;
 
@@ -881,7 +877,6 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
 	if (!upstream)
 		goto done;
 
-	free_commit_list(merge_bases);
 	merge_bases = get_merge_bases(upstream, head);
 	if (!merge_bases || merge_bases->next)
 		goto done;
@@ -896,6 +891,20 @@ done:
 	return res && is_linear_history(onto, head);
 }
 
+static void fill_branch_base(struct rebase_options *options,
+			    struct object_id *branch_base)
+{
+	struct commit_list *merge_bases = NULL;
+
+	merge_bases = get_merge_bases(options->onto, options->orig_head);
+	if (!merge_bases || merge_bases->next)
+		oidcpy(branch_base, null_oid());
+	else
+		oidcpy(branch_base, &merge_bases->item->object.oid);
+
+	free_commit_list(merge_bases);
+}
+
 static int parse_opt_am(const struct option *opt, const char *arg, int unset)
 {
 	struct rebase_options *opts = opt->value;
@@ -1660,6 +1669,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		if (!options.onto)
 			die(_("Does not point to a valid commit '%s'"),
 				options.onto_name);
+		fill_branch_base(&options, &branch_base);
 	}
 
 	if (options.fork_point > 0)
@@ -1689,13 +1699,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	 * Check if we are already based on onto with linear history,
 	 * in which case we could fast-forward without replacing the commits
 	 * with new commits recreated by replaying their changes.
-	 *
-	 * Note that can_fast_forward() initializes branch_base, so we have to
-	 * call it before checking allow_preemptive_ff.
 	 */
-	if (can_fast_forward(options.onto, options.upstream, options.restrict_revision,
-		    options.orig_head, &branch_base) &&
-	    allow_preemptive_ff) {
+	if (allow_preemptive_ff &&
+	    can_fast_forward(options.onto, options.upstream, options.restrict_revision,
+			     options.orig_head, &branch_base)) {
 		int flag;
 
 		if (!(options.flags & REBASE_FORCE)) {