SelectMany() 无法扁平化词典类型

例子，把一个 List<Dictionary<string, string>> 展开为 KeyValuePair 列表，结果会报错：
Error：解析表达式错误。
Oops! No applicable member has been found for the expression. The error occurred for expression "." at position 315 near ".SelectMany(d => d)

var dictList = new List<Dictionary<string, string>>
{
    new Dictionary<string, string>()
    {
        {"key1", "value1"},
        {"key2", "value2"}
    },
    new Dictionary<string, string>()
    {
        {"key3", "value3"},
        {"key4", "value4"}
    }
};


var mergedDict = dictList
	.SelectMany(d => d)
	.ToDictionary(kv => kv.Key, kv => kv.Value);