diff --git a/straindesign/strainDesignMILP.py b/straindesign/strainDesignMILP.py
index 8c34a5a..ea93ebc 100644
--- a/straindesign/strainDesignMILP.py
+++ b/straindesign/strainDesignMILP.py
@@ -344,6 +344,7 @@ def compute_optimal(self, **kwargs):
         else:
             logging.info('Time limit reached.')
         # Translate solutions into dict
+        sd_dict = []
         for sol in sols:
             sd_dict += [self.sd2dict(sol, self.show_no_ki)]
         return self.build_sd_solution(sd_dict, status, BEST)