diff --git a/ldvspec/lofardata/forms.py b/ldvspec/lofardata/forms.py
index 2aea9a0d2050122bda7a6b53ac19db2088c7f6ea..faa5152bbb8f74145d54a9e5a9c728ff61583aa7 100644
--- a/ldvspec/lofardata/forms.py
+++ b/ldvspec/lofardata/forms.py
@@ -23,7 +23,7 @@ class WorkSpecificationForm(ModelForm):
         self.cleaned_data = super().clean()
         self.cleaned_data["filters"] = self._extract_filters()
         if not 'obs_id' in self.cleaned_data["filters"]:
-            raise ValidationError(["You have forgotten about the 'obs_id'"])
+            raise ValidationError(["obs_id: This field is required."])
 
         return self.cleaned_data