diff --git a/pysb/importers/boolean.py b/pysb/importers/boolean.py
index f39cda12..3091b341 100644
--- a/pysb/importers/boolean.py
+++ b/pysb/importers/boolean.py
@@ -79,7 +79,7 @@ def __init__(self, filename, format='BooleanNet', mode='GSP', force=False):
             nfired_pat = []
         #~~~~~
         for name,state in self.initial_states.items():
-            rank = self.function_ranks.get(name)
+            rank = self.function_ranks.get(name, 1)
             # create monomer
             mon_sites = ['state']
             mon_site_states = {'state' : ['False','True']}