diff --git a/ippisite/ippidb/models.py b/ippisite/ippidb/models.py
index 14aa22b4f20263ea78e148886de9dc3d5364ce8a..9e5c066e4a3434259c7364f980780a76cf09f5e3 100644
--- a/ippisite/ippidb/models.py
+++ b/ippisite/ippidb/models.py
@@ -297,7 +297,7 @@ class ProteinDomainComplex(models.Model):
     Protein-Domain association
     """
     protein = models.ForeignKey('Protein', models.CASCADE)
-    domain = models.ForeignKey('Domain', models.CASCADE)
+    domain = models.ForeignKey('Domain', models.CASCADE, null=True, blank=True)
     ppc_copy_nb = models.IntegerField(
         'Number of copies of the protein in the complex')