Error: no operator "==" matches these operands

[taymaxi]

I'm trying to do a binary search on a vector of strings and keep getting this error. I just can't for the life of me figure out what is wrong, so if anyone could help me out that'd be really amazing.

This is the PhoneEntry header file with the class declaration:

using namespace std;
#include<string>
#include<iostream>
#include<fstream>
#include<vector>
#include"phoneNumber.h"
class PhoneEntry
{
private:
	PhoneNumber eNumber;
	string firstName, 
		   lastName;
	void writeDots(ostream&, int) const;
public:
	istream& readEntry(istream&);
	ostream& writeEntry(ostream&) const;
	bool greaterAlpha(const PhoneEntry &);
	void mySort(vector<PhoneEntry>&, int);
	PhoneEntry key(string, string);
};

And here's the part of the program that I'm having issues with:

int binarySearch(PhoneEntry keyP, vector<PhoneEntry> entryNames)
{
	int low = 0, mid, high = entryNames.size()-1;

	while (low <= high)
	{
		mid = (low + high) / 2;
		if (keyP.greaterAlpha(entryNames[mid])) 
                //greaterAlpha compares the two strings and returns true or false based on alphabetical order
			high = mid - 1;
		else if (!keyP.greaterAlpha(entryNames[mid]))
			low = high + 1;
		else
			low = high + 1;
	}

	if (keyP == entryNames[mid]) //This is the line where I keep getting the error
		return mid;
	else
		return (mid * -1);
};