Static assert on size of static const member

I'm trying to put a static assert on the size of static const unordered_map member. But I get an error saying non-const condition for static assertion. Could someone help?

#include<unordered_map>
#include<string>

using namespace std;    

class A{
    public:
    static const unordered_map<string,string> strMap;
};

const unordered_map<string,string> A::strMap ={{"key","value"}};

int main() {
static_assert(A::strMap.size() == 1, "sizes don't match");
}