diff --git a/src/build.ts b/src/build.ts
index b818043..d053b07 100644
--- a/src/build.ts
+++ b/src/build.ts
@@ -4,17 +4,18 @@ import { build_markdown_from_json_schema } from '@doc-utils/jsonschema2markdown'
 import { render_markdown_to_html, render_markdown_to_html_inline_sync } from '@doc-utils/markdown2html';
 
 import { glob } from 'glob';
-import { Config } from './conf';
+import { DateTime } from 'luxon';
+import { stringify as to_yaml } from 'yaml';
+
 import { promises as fs } from 'fs';
 import { dirname, join as path_join } from 'path';
-import { build_env_scope } from './env';
-import { load_layout, Context, render_template, load_partials, load_extras, FrontMatter } from './template';
-import { DateTime } from 'luxon';
-import assert = require('assert');
-import { load_themes, render_theme_css_properties } from './themes';
+
 import { icons } from './icons';
+import { Config } from './conf';
+import { build_env_scope } from './env';
+import { load_themes, render_theme_css_properties } from './themes';
 import { mkdirp, read_json, read_text, read_yaml, write_text } from './fs';
-import { stringify as to_yaml } from 'yaml';
+import { load_layout, Context, render_template, load_partials, load_extras, FrontMatter } from './template';
 
 interface BuildState {
 	conf: Config;
@@ -364,7 +365,10 @@ async function render_json_schema(state: BuildState, schema: unknown, out_file:
 // ===== Helpers =====
 
 async function map_input_file_to_output_file(state: BuildState, in_file: string, remove_exts?: string[], add_ext?: string) {
-	assert(in_file.startsWith(state.conf.input.root), 'input file expected to be inside input root');
+	if (! in_file.startsWith(state.conf.input.root)) {
+		throw new Error('input file expected to be inside input root');
+	}
+	
 	let out_file = path_join(state.conf.output.root, in_file.slice(state.conf.input.root.length));
 
 	if (remove_exts) {