Managed C++/CLI

error C2451: conditional expression of type 'class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >' is illegal
        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

...
packetCount = fillPackets(inputString);
cout << "Filled " << packetCount << " packets." << endl;
...

int fillPackets(string inputString) {
	string aPacket;
	int packetCount = 0;
	int x = 0;

	cout << "Filling packets...";
	
	while (aPacket = getNextPacket(inputString)) { <code><-- ERROR!</code>
		packets[x++] = new packet(aPacket);
		packetCount++;
	}
		
	cout << "done." << endl;
	return packetCount;
}

string getNextPacket(string packetString) {
	string aPacket;
	static string::const_iterator theIterator = packetString.begin();
	int start = 0, end = 0;

	// search for the pattern \x00\x00\x00\x??\x00\x00\x00
	// - end of ethernet preamble
	while (theIterator != packetString.end() &&
		*theIterator != (char) 0 &&
		*(theIterator + 1) != (char) 0 &&
		*(theIterator + 2) != (char) 0 &&
		*(theIterator + 3) == (char) 0 &&
		*(theIterator + 4) != (char) 0 &&
		*(theIterator + 5) != (char) 0 &&
		*(theIterator + 6) != (char) 0) {

		++theIterator;
		start++;
	}

	if (theIterator == packetString.end())
		return NULL;
			
	theIterator += 7;
	start += 7;

	// search for the pattern \x00\x00\x00\x??\x00\x00\x00
	// - end of ethernet preamble
	while (theIterator != packetString.end() &&
		*theIterator != (char) 0 &&
		*(theIterator + 1) != (char) 0 &&
		*(theIterator + 2) != (char) 0 &&
		*(theIterator + 3) == (char) 0 &&
		*(theIterator + 4) != (char) 0 &&
		*(theIterator + 5) != (char) 0 &&
		*(theIterator + 6) != (char) 0) {	
			
		++theIterator;
		end++;
	}

	if (theIterator == packetString.end())
		return NULL;

	aPacket = packetString.substr(start, end);
	return aPacket;
}