C++: no matching function for call: why is an empty constructor needed?

When I try to compile the following code:

class a {

  int i;

  public :
  a(int);
};

class b {
  a mya;  
  int j;

  public:
  b(int);

};

a::a(int i2) {
  i=i2;
}

b::b(int i2) {
  mya=a(i2); 
  j=2*i2;
}

int main() {

}

I get the following errors:

prog.cpp:21:12: error: no matching function for call to ‘a::a()

 b::b(int i2) {
            ^
prog.cpp:17:1: note: candidate: ‘a::a(int)

 a::a(int i2) {
 ^
prog.cpp:17:1: note:   candidate expects 1 argument, 0 provided
prog.cpp:1:7: note: candidate: ‘constexpr a::a(const a&)’
 class a {
       ^
prog.cpp:1:7: note:   candidate expects 1 argument, 0 provided
prog.cpp:1:7: note: candidate: ‘constexpr a::a(a&&)

prog.cpp:1:7: note:   candidate expects 1 argument, 0 provided

It seems that a constructor with no argument for the class a is expected. I do not understand why, the only time I create a object of type a, I call the constructor which takes an int as argument.

I understand that the solution would be to add a constructor without arguments for a. But why ?

Thank you for your answers, best regards,

Jerome