diff --git a/lookup.py b/lookup.py
index fc3a2f6..62fde8c 100755
--- a/lookup.py
+++ b/lookup.py
@@ -1,11 +1,15 @@
 #!/usr/bin/python3
 """Reverse geocode: convert lat/lon to Wikidata item & Wikimedia Commons category."""
 
+import inspect
 import random
 import socket
+import sys
+import traceback
 import typing
 
 import sqlalchemy.exc
+import werkzeug.debug.tbtools
 from flask import Flask, jsonify, redirect, render_template, request, url_for
 from sqlalchemy.orm.query import Query
 from werkzeug.wrappers import Response
@@ -24,6 +28,33 @@ Tags = typing.Mapping[str, str]
 logging_enabled = True
 
 
+@app.errorhandler(werkzeug.exceptions.InternalServerError)
+def exception_handler(e: werkzeug.exceptions.InternalServerError) -> tuple[str, int]:
+    """Handle exception."""
+    exec_type, exc_value, current_traceback = sys.exc_info()
+    assert exc_value
+    tb = werkzeug.debug.tbtools.DebugTraceback(exc_value)
+
+    summary = tb.render_traceback_html(include_title=False)
+    exc_lines = "".join(tb._te.format_exception_only())
+
+    last_frame = list(traceback.walk_tb(current_traceback))[-1][0]
+    last_frame_args = inspect.getargs(last_frame.f_code)
+
+    return (
+        render_template(
+            "show_error.html",
+            plaintext=tb.render_traceback_text(),
+            exception=exc_lines,
+            exception_type=tb._te.exc_type.__name__,
+            summary=summary,
+            last_frame=last_frame,
+            last_frame_args=last_frame_args,
+        ),
+        500,
+    )
+
+
 def get_random_lat_lon() -> tuple[float, float]:
     """Select random lat/lon within the UK."""
     south, east = 50.8520, 0.3536
diff --git a/static/css/exception.css b/static/css/exception.css
new file mode 100644
index 0000000..1f141c5
--- /dev/null
+++ b/static/css/exception.css
@@ -0,0 +1,78 @@
+div.debugger { text-align: left; padding: 12px; margin: auto;
+               background-color: white; }
+div.detail { cursor: pointer; }
+div.detail p { margin: 0 0 8px 13px; font-size: 14px; white-space: pre-wrap;
+               font-family: monospace; }
+div.explanation { margin: 20px 13px; font-size: 15px; color: #555; }
+div.footer   { font-size: 13px; text-align: right; margin: 30px 0;
+               color: #86989B; }
+
+h2           { font-size: 16px; margin: 1.3em 0 0.0 0; padding: 9px;
+               background-color: #11557C; color: white; }
+h2 em, h3 em { font-style: normal; color: #A5D6D9; font-weight: normal; }
+
+div.traceback, div.plain { border: 1px solid #ddd; margin: 0 0 1em 0; padding: 10px; }
+div.plain p      { margin: 0; }
+div.plain textarea,
+div.plain pre { margin: 10px 0 0 0; padding: 4px;
+                background-color: #E8EFF0; border: 1px solid #D3E7E9; }
+div.plain textarea { width: 99%; height: 300px; }
+div.traceback h3 { font-size: 1em; margin: 0 0 0.8em 0; }
+div.traceback ul { list-style: none; margin: 0; padding: 0 0 0 1em; }
+div.traceback h4 { font-size: 13px; font-weight: normal; margin: 0.7em 0 0.1em 0; }
+div.traceback pre { margin: 0; padding: 5px 0 3px 15px;
+                    background-color: #E8EFF0; border: 1px solid #D3E7E9; }
+div.traceback .library .current { background: white; color: #555; }
+div.traceback .expanded .current { background: #E8EFF0; color: black; }
+div.traceback pre:hover { background-color: #DDECEE; color: black; cursor: pointer; }
+div.traceback div.source.expanded pre + pre { border-top: none; }
+
+div.traceback span.ws { display: none; }
+div.traceback pre.before, div.traceback pre.after { display: none; background: white; }
+div.traceback div.source.expanded pre.before,
+div.traceback div.source.expanded pre.after {
+    display: block;
+}
+
+div.traceback div.source.expanded span.ws {
+    display: inline;
+}
+
+div.traceback blockquote { margin: 1em 0 0 0; padding: 0; white-space: pre-line; }
+div.traceback img { float: right; padding: 2px; margin: -3px 2px 0 0; display: none; }
+div.traceback img:hover { background-color: #ddd; cursor: pointer;
+                          border-color: #BFDDE0; }
+div.traceback pre:hover img { display: block; }
+div.traceback cite.filename { font-style: normal; color: #3B666B; }
+
+pre.console { border: 1px solid #ccc; background: white!important;
+              color: black; padding: 5px!important;
+              margin: 3px 0 0 0!important; cursor: default!important;
+              max-height: 400px; overflow: auto; }
+pre.console form { color: #555; }
+pre.console input { background-color: transparent; color: #555;
+                    width: 90%; font-family: 'Consolas', 'Deja Vu Sans Mono',
+                    'Bitstream Vera Sans Mono', monospace; font-size: 14px;
+                     border: none!important; }
+
+span.string { color: #30799B; }
+span.number { color: #9C1A1C; }
+span.help   { color: #3A7734; }
+span.object { color: #485F6E; }
+span.extended { opacity: 0.5; }
+span.extended:hover { opacity: 1; }
+a.toggle { text-decoration: none; background-repeat: no-repeat;
+           background-position: center center;
+           background-image: url(?__debugger__=yes&cmd=resource&f=more.png); }
+a.toggle:hover { background-color: #444; }
+a.open { background-image: url(?__debugger__=yes&cmd=resource&f=less.png); }
+
+div.traceback pre, div.console pre {
+    white-space: pre-wrap;       /* css-3 should we be so lucky... */
+    white-space: -moz-pre-wrap;  /* Mozilla, since 1999 */
+    white-space: -pre-wrap;      /* Opera 4-6 ?? */
+    white-space: -o-pre-wrap;    /* Opera 7 ?? */
+    word-wrap: break-word;       /* Internet Explorer 5.5+ */
+    _white-space: pre;           /* IE only hack to re-specify in
+                                 addition to word-wrap  */
+}
diff --git a/templates/show_error.html b/templates/show_error.html
new file mode 100644
index 0000000..dd79c3c
--- /dev/null
+++ b/templates/show_error.html
@@ -0,0 +1,23 @@
+{% extends "base.html" %}
+
+{% block style %}
+<link rel="stylesheet" href="{{url_for('static', filename='css/exception.css')}}" />
+{% endblock %}
+
+{% block content %}
+<div class="p-2">
+
+<h1>Software error: {{ exception_type }}</h1>
+<div>
+  <pre>{{ exception }}</pre>
+</div>
+
+<h2 class="traceback">Traceback <em>(most recent call last)</em></h2>
+{{ summary | safe }}
+
+<p>Error in function "{{ last_frame.f_code.co_name }}": {{ last_frame_args | pprint }}</p>
+<pre>{{ last_frame.f_locals | pprint }}</pre>
+
+</div>
+
+{% endblock %}