error while drawing lines from houghlines()

hi guys, im new to openCv and image processing in general and i need to draw the lines in real time from a camera input. it working fine until i try to draw in the original image the lines detected with houghlines(). here is my code:

int main(int argc, char* argv[])
{	
	Mat input;
	Mat HSV;
	Mat threshold;
	Mat CannyThresh;
	Mat HL;

	//video capture object to acquire webcam feed
	cv::VideoCapture capture;
	capture.open(0);
	capture.set(CV_CAP_PROP_FRAME_WIDTH, 640);
	capture.set(CV_CAP_PROP_FRAME_HEIGHT, 480);

	//start an infinite loop where webcam feed is copied to cameraFeed matrix
	//all operations will be performed within this loop

	while (true){ 

		capture.read(input);
		cvtColor(input, HSV, COLOR_BGR2HSV); //hsv
		inRange(HSV, Scalar(H_MIN, S_MIN, V_MIN), Scalar(H_MAX, S_MAX, V_MAX), threshold);//thershold
		MorphOps(threshold);//morph operations on threshold image
		Canny(threshold, CannyThresh, 100, 50); //canny edge detection

		std::vector<Vec4i> lines;
		HoughLines(CannyThresh, lines, 1, CV_PI/180, 150, 0, 0 );

	    for( size_t i = 0; i < lines.size(); i++ )
		{
			float rho = lines[i][0], theta = lines[i][1];
			Point pt1, pt2;
			double a = cos(theta), b = sin(theta);
			double x0 = a*rho, y0 = b*rho;
			pt1.x = cvRound(x0 + 1000*(-b));
			pt1.y = cvRound(y0 + 1000*(a));
			pt2.x = cvRound(x0 - 1000*(-b));
			pt2.y = cvRound(y0 - 1000*(a));
			line( input, pt1, pt2, Scalar(0,0,255), 3, CV_AA);
		} 

		imshow("camera", input);
		waitKey(30);
		
	}
return 0;
}

im getting the following exception:

First-chance exception at 0x74DB3E28 in Proyecto0.1.exe: Microsoft C++ exception: cv::Exception at memory location 0x00D6F47C.

i know the error is happening in the

for( size_t i = 0; i < lines.size(); i++ )

line because if i comment everything inside the for loop, i still get the same error, but if i remove the whole for-loop, it doesnt throws the exception. any ideas of what may be happening?