ValueError: invalid literal for int() with base 10: 'Pallet_Truck'. Where is the mistake?

Question

Django

Misters, help to understand an error. I understand its essence - an attempt to convert a string to a number. However, in the foreseeable code, no operations with numbers occur. Where is the "dog" buried?
URLS.PY

from django.urls import path
from . import views
from .views import tech_type_list

urlpatterns = [
    path('', views.tech_list, name='tech_list'),
    path('<tech_type>/', tech_type_list, name='tech_type'),
]

Models.py

from django.db import models


class TechType(models.Model):
    type_name = models.CharField(max_length=100, verbose_name='Вид техники', unique=True)

    def __str__(self):
        return self.type_name


class TechUnit(models.Model):
    inner_id = models.CharField(max_length=100, verbose_name='Внутренний номер', unique=True)
    tech_type = models.ForeignKey(TechType, on_delete=models.CASCADE, verbose_name='Вид техники')

    def __str__(self):
        return self.inner_id

Views.py

from django.shortcuts import render
from .models import TechType, TechUnit
from django.shortcuts import render, get_object_or_404


def tech_list(request):
    tech_type = TechType.objects.order_by('type_name')
    return render(request, 'checklist/tech_list_page.html', {'tech_type': tech_type})


def tech_type_list(request, tech_type):
    template = 'checklist/tech_detail.html'
    tech_list = TechUnit.objects.filter(tech_type=tech_type)
    context = {
        'tech_list': tech_list,
    }
    return render(request, template, context)

Well, actually the template in which I'm trying to display all this

{% extends 'checklist/base.html' %}

{% block content %}
<h1>Единицы техники типа {{ tech_list.tech_type }}</h1>
<ul>
    {% for item in tech_list %}
    <li>{{ item }}</li>
    {% endfor %}
</ul>
{% endblock %}

If you give an instance of the tech_type class a name consisting of numbers, there are no errors. If you enter letters, ValueError: invalid literal for int() with base 10 appears. Where is Django trying to convert letters to numbers?