将RichTextBox字符串转换为Int时出现FormatException

Question

这是整个代码：

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

namespace Calculator
{
	public partial class MainForm : Form
	{
		public MainForm()
		{
			InitializeComponent();
			richTextBox1.ReadOnly = true;
		}
		
		//inputs
		void Button1Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("1");
		}
		void Button2Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("2");
		}
		void Button3Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("3");
		}
		void Button6Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("4");
		}
		void Button5Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("5");
		}
		void Button4Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("6");
		}
		void Button9Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("7");
		}
		void Button8Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("8");
		}
		void Button7Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("9");
		}
		void Button10Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("0");
		}
		void Button14Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("+");
		}
		void Button13Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("-");
		}
		void Button12Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("X");
		}
		void Button11Click(object sender, EventArgs e)
		{
			richTextBox1.AppendText("/");
		}
		void Button15Click(object sender, EventArgs e)
		{
			String fetch = richTextBox1.Text;
			String convert = fetch.Replace('X', '*');
			try
			{
				int equ = Int32.Parse(convert);
			}
			catch (FormatException exe)
			{
				MessageBox.Show(exe.Message);
			}
		}
		
	}
}

这是引发异常的部分：

void Button15Click(object sender, EventArgs e)
		{
			String fetch = richTextBox1.Text;
			String convert = fetch.Replace('X', '*');
			try
			{
				int equ = Int32.Parse(convert);
			}
			catch (FormatException exe)
			{
				MessageBox.Show(exe.Message);
			}
		}

由于某种原因，Int32.parse无法将“转换”字符串更改为整数，而是抛出“输入字符串格式不正确”。我不知道为什么。我也尝试过使用Convert.Int32（），但是它仍然会引发相同的异常。