diff --git a/src/simsopt/mhd/spec.py b/src/simsopt/mhd/spec.py
index 690eb9bdb..2e4d91404 100644
--- a/src/simsopt/mhd/spec.py
+++ b/src/simsopt/mhd/spec.py
@@ -1285,7 +1285,7 @@ def as_spec(self):
         
         @as_spec.setter
         def as_spec(self, array):
-            if array.shape != [2*self.mntor+1, 2*self.mmpol+1]:
+            if array.shape != (2*self._mntor+1, 2*self._mmpol+1):
                 raise ValueError('Array size is not consistent witn mmpol and mntor')
             self._array = array