diff --git a/src/main/java/de/kosit/validationtool/daemon/BaseHandler.java b/src/main/java/de/kosit/validationtool/daemon/BaseHandler.java
index 4a7fe54..9aaae1c 100644
--- a/src/main/java/de/kosit/validationtool/daemon/BaseHandler.java
+++ b/src/main/java/de/kosit/validationtool/daemon/BaseHandler.java
@@ -29,10 +29,11 @@ abstract class BaseHandler implements HttpHandler {
         os.close();
     }
 
-    protected static void error(final HttpExchange httpExchange, final int statusCode, final String message) throws IOException {
+    protected static void error(final HttpExchange exchange, final int statusCode, final String message) throws IOException {
         final byte[] bytes = message.getBytes();
-        httpExchange.sendResponseHeaders(statusCode, bytes.length);
-        final OutputStream os = httpExchange.getResponseBody();
+        exchange.getResponseHeaders().add("Content-Type", "text/plain");
+        exchange.sendResponseHeaders(statusCode, bytes.length);
+        final OutputStream os = exchange.getResponseBody();
         os.write(bytes);
         os.close();
     }
diff --git a/src/test/java/de/kosit/validationtool/daemon/GuiHandlerIT.java b/src/test/java/de/kosit/validationtool/daemon/GuiHandlerIT.java
new file mode 100644
index 0000000..bd62201
--- /dev/null
+++ b/src/test/java/de/kosit/validationtool/daemon/GuiHandlerIT.java
@@ -0,0 +1,16 @@
+package de.kosit.validationtool.daemon;
+
+import io.restassured.http.ContentType;
+import org.junit.Test;
+
+import static io.restassured.RestAssured.given;
+
+public class GuiHandlerIT extends BaseIT {
+
+    @Test
+    public void checkGui() {
+        given().when().get("/").then().statusCode(200).and().contentType(ContentType.HTML);
+        given().when().get("/README.md").then().statusCode(200).and().contentType("text/markdown");
+        given().when().get("/unknown.md").then().statusCode(404).and().contentType(ContentType.TEXT);
+    }
+}