diff --git a/prolefeeder.py b/prolefeeder.py
index 7b701ac..d0cda23 100644
--- a/prolefeeder.py
+++ b/prolefeeder.py
@@ -34,7 +34,9 @@ class DownloadForm(FlaskForm):
     length = IntegerField('Länge',
                           validators=[
                               InputRequired(),
-                              NumberRange(min=1, max=app.config['MAX_LENGTH'])])
+                              NumberRange(
+                                  min=1, max=app.config['MAX_LENGTH'],
+                                  message="Länge muss zwischen %(min)s and %(max)s Minuten sein")])
     submit = SubmitField('Download')
 
 
diff --git a/requirements.txt b/requirements.txt
index c006d77..3e77d9d 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,3 +2,4 @@ flask
 bootstrap-flask
 python-dotenv
 flask-wtf >= 1.0.0
+WTForms >= 3.0.0