diff --git a/tests/test_form_pages.py b/tests/test_form_pages.py
index 81fa9d27c..49c3eb8ee 100644
--- a/tests/test_form_pages.py
+++ b/tests/test_form_pages.py
@@ -4238,6 +4238,25 @@ def test_form_text_field_submit(pub):
     resp = resp.forms[0].submit('submit')
     assert 'Check values then click submit.' in resp.body
 
+def test_unknown_datasource(pub):
+    formdef = create_formdef()
+    formdef.fields = [
+        fields.StringField(id='1', label='string',
+                varname='string', required=False, data_source={'type': 'foobar'}),
+        fields.ItemField(id='2', label='item',
+                varname='item', required=False, data_source={'type': 'foobar'}),
+        fields.ItemsField(id='3', label='items',
+                varname='items', required=False, data_source={'type': 'foobar'}),
+    ]
+
+    formdef.store()
+    data_class = formdef.data_class()
+    data_class.wipe()
+    resp = get_app(pub).get('/test/')
+    formdef.data_class().wipe()
+    resp = resp.forms[0].submit('submit') # should go straight to validation
+    assert 'Check values then click submit.' in resp.body
+
 def test_form_items_datasource(pub):
     formdef = create_formdef()
     formdef.fields = [fields.ItemsField(id='1', label='items',
diff --git a/wcs/data_sources.py b/wcs/data_sources.py
index 5bafd4d06..04bcf1729 100644
--- a/wcs/data_sources.py
+++ b/wcs/data_sources.py
@@ -249,6 +249,12 @@ def get_object(data_source):
     return NamedDataSource.get_by_slug(ds_type)
 
 
+class StubNamedDataSource(object):
+    type = 'formula'
+    data_source = {'type': 'formula', 'value': []}
+    cache_duration = None
+
+
 class NamedDataSource(XmlStorableObject):
     _names = 'datasources'
     _xml_tagname = 'datasources'
@@ -330,7 +336,8 @@ class NamedDataSource(XmlStorableObject):
         objects = [x for x in cls.select() if x.slug == slug]
         if objects:
             return objects[0]
-        raise KeyError("data source '%s' does not exist" % slug)
+        get_logger().warn("data source '%s' does not exist" % slug)
+        return StubNamedDataSource()
 
     def get_json_query_url(self):
         url = self.data_source.get('value').strip()
diff --git a/wcs/formdef.py b/wcs/formdef.py
index 3ba4219ec..5d35fafed 100644
--- a/wcs/formdef.py
+++ b/wcs/formdef.py
@@ -1010,9 +1010,8 @@ class FormDef(StorableObject):
         for field in formdef.fields:
             data_source = getattr(field, 'data_source', None)
             if data_source:
-                try:
-                    data_sources.get_real(data_source)
-                except KeyError:
+                if isinstance(data_sources.get_object(data_source),
+                              data_sources.StubNamedDataSource):
                     unknown_datasources.add(data_source.get('type'))
         if unknown_datasources:
             raise FormdefImportError(N_('Unknown datasources'),