diff --git a/lualib-src/lua-bson.c b/lualib-src/lua-bson.c
index 52baa64c..9071f56d 100644
--- a/lualib-src/lua-bson.c
+++ b/lualib-src/lua-bson.c
@@ -243,12 +243,16 @@ append_key(struct bson *bs, int type, const char *key, size_t sz) {
 	write_string(bs, key, sz);
 }
 
+static inline int
+is_32bit(int64_t v) {
+	return v >= INT32_MIN && v <= INT32_MAX;
+}
+
 static void
 append_number(struct bson *bs, lua_State *L, const char *key, size_t sz) {
 	if (lua_isinteger(L, -1)) {
 		int64_t i = lua_tointeger(L, -1);
-		int si = i >> 31;
-		if (si == 0 || si == -1) {
+		if (is_32bit(i)) {
 			append_key(bs, BSON_INT32, key, sz);
 			write_int32(bs, i);
 		} else {