Index: django/contrib/gis/utils/layermapping.py
===================================================================
--- django/contrib/gis/utils/layermapping.py	(revision 17150)
+++ django/contrib/gis/utils/layermapping.py	(working copy)
@@ -295,7 +295,13 @@
             elif isinstance(model_field, models.base.ModelBase):
                 # The related _model_, not a field was passed in -- indicating
                 # another mapping for the related Model.
-                val = self.verify_fk(feat, model_field, ogr_name)
+                try:
+                    val = self.verify_fk(feat, model_field, ogr_name)
+                except MissingForeignKey:
+                    if self.model._meta.get_field(field_name).null:
+                        val = None
+                    else:
+                        raise
             else:
                 # Otherwise, verify OGR Field type.
                 val = self.verify_ogr_field(feat[ogr_name], model_field)