diff --git a/src/core/create_routes.ts b/src/core/create_routes.ts
index 4515076..0b6c3e8 100644
--- a/src/core/create_routes.ts
+++ b/src/core/create_routes.ts
@@ -11,6 +11,10 @@ export default function create_routes({ files } = { files: glob.sync('**/*.*', {
 				throw new Error(`Invalid route ${file} — parameters must be separated`);
 			}
 
+			if (file === '4xx.html' || file === '5xx.html') {
+				throw new Error('As of Sapper 0.14, 4xx.html and 5xx.html should be replaced with _error.html');
+			}
+
 			const base = file.replace(/\.[^/.]+$/, '');
 			const parts = base.split('/'); // glob output is always posix-style
 			if (parts[parts.length - 1] === 'index') parts.pop();
diff --git a/test/unit/create_routes.test.js b/test/unit/create_routes.test.js
index f299464..95eb116 100644
--- a/test/unit/create_routes.test.js
+++ b/test/unit/create_routes.test.js
@@ -280,4 +280,20 @@ describe('create_routes', () => {
 			});
 		}, /Invalid route \[foo\]\[bar\]\.js — parameters must be separated/);
 	});
+
+	it('errors on 4xx.html', () => {
+		assert.throws(() => {
+			create_routes({
+				files: ['4xx.html']
+			});
+		}, /As of Sapper 0.14, 4xx.html and 5xx.html should be replaced with _error.html/);
+	});
+
+	it('errors on 5xx.html', () => {
+		assert.throws(() => {
+			create_routes({
+				files: ['5xx.html']
+			});
+		}, /As of Sapper 0.14, 4xx.html and 5xx.html should be replaced with _error.html/);
+	});
 });
\ No newline at end of file