diff --git a/fissa/core.py b/fissa/core.py
index 831f25f9..8e173f97 100644
--- a/fissa/core.py
+++ b/fissa/core.py
@@ -325,6 +325,7 @@ def separation_prep(self, redo=False):
 
             # predefine data structures
             raw = [[None for t in range(self.nTrials)] for c in range(nCell)]
+            raw = np.asarray(raw)
             roi_polys = np.copy(raw)
 
             # store results
@@ -407,6 +408,7 @@ def separate(self, redo_prep=False, redo_sep=False):
             # predefine data structures
             sep = [[None for t in range(self.nTrials)]
                    for c in range(self.nCell)]
+            sep = np.asarray(sep)
             result = np.copy(sep)
             mixmat = np.copy(sep)
             info = np.copy(sep)
@@ -499,6 +501,7 @@ def calc_deltaf(self, freq, use_raw_f0=True, across_trials=True):
         """
         deltaf_raw = [[None for t in range(self.nTrials)]
                       for c in range(self.nCell)]
+        deltaf_raw = np.asarray(deltaf_raw)
         deltaf_result = np.copy(deltaf_raw)
 
         # loop over cells