Constructor Overloading with Default Parameters

I accidentally overloaded a constructor in C# as follows:

public MyClass(string myString) 
{
    // Some code goes here 
}

public MyClass(string myString, bool myParameter = false) 
{
   // Some different code here
}

With this code my project compiled fine. If I call the constructor with just a string argument, how does C# decide which constructor I want to use? Why is this functionality syntactically allowed?