这句错误是什么意思？

[pwtitle]

//重载[]
char &MyString::operator[](int subscript)
{
if( (subscript>=0)&&(subscript<=size) ){
return this->str[subscript];
}
}


D:\myc\StringADT\mystring.cpp In member function `char& MyString::operator[](int)':
99 D:\myc\StringADT\mystring.cpp [Warning] control reaches end of non-void function

[pwtitle]

环境，dev-c++ 4.9.10

[xuzuning]

char &MyString::operator[](int subscript)
{
if( (subscript>=0)&&(subscript<=size) ){
return this->str[subscript];
}
return '';// <== 这里缺少返回语句
}