diff --git a/modelseedpy/core/msbuilder.py b/modelseedpy/core/msbuilder.py
index e376ae0b..f3514f7f 100644
--- a/modelseedpy/core/msbuilder.py
+++ b/modelseedpy/core/msbuilder.py
@@ -869,7 +869,7 @@ def build(
         biomass_reactions = []
         for rxn_biomass in self.template.biomasses:
             reaction = rxn_biomass.build_biomass(
-                cobra_model, "0", biomass_classic, biomass_gc
+                cobra_model, index, biomass_classic, biomass_gc
             )
             for m in reaction.metabolites:
                 if "modelseed_template_id" in m.notes: