diff --git a/builtin-diff.c b/builtin-diff.c
index 2e51f408f9..62b483bd5f 100644
--- a/builtin-diff.c
+++ b/builtin-diff.c
@@ -405,17 +405,32 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 		result = builtin_diff_index(&rev, argc, argv);
 	else if (ents == 2)
 		result = builtin_diff_tree(&rev, argc, argv, ent);
-	else if ((ents == 3) && (ent[0].item->flags & UNINTERESTING)) {
-		/* diff A...B where there is one sane merge base between
-		 * A and B.  We have ent[0] == merge-base, ent[1] == A,
-		 * and ent[2] == B.  Show diff between the base and B.
+	else if (ent[0].item->flags & UNINTERESTING) {
+		/*
+		 * Perhaps the user gave us A...B, which expands
+		 * to a list of negative merge bases followed by
+		 * A (symmetric-left) and B?  Let's make sure...
 		 */
-		ent[1] = ent[2];
+		for (i = 1; i < ents; i++)
+			if (!(ent[i].item->flags & UNINTERESTING))
+				break;
+		if (ents != i + 2 ||
+		    (ent[i+1].item->flags & UNINTERESTING) ||
+		    (!(ent[i].item->flags & SYMMETRIC_LEFT)) ||
+		    (ent[i+1].item->flags & SYMMETRIC_LEFT))
+			die("what do you mean by that?");
+		/*
+		 * diff A...B where there is at least one merge base
+		 * between A and B.  We have ent[0] == merge-base,
+		 * ent[ents-2] == A, and ent[ents-1] == B.  Show diff
+		 * between the base and B.  Note that we pick one
+		 * merge base at random if there are more than one.
+		 */
+		ent[1] = ent[ents-1];
 		result = builtin_diff_tree(&rev, argc, argv, ent);
-	}
-	else
+	} else
 		result = builtin_diff_combined(&rev, argc, argv,
-					     ent, ents);
+					       ent, ents);
 	result = diff_result_code(&rev.diffopt, result);
 	if (1 < rev.diffopt.skip_stat_unmatch)
 		refresh_index_quietly();