diff --git a/pynonthermal/spencerfano.py b/pynonthermal/spencerfano.py
index a7ab84a..dd351a4 100644
--- a/pynonthermal/spencerfano.py
+++ b/pynonthermal/spencerfano.py
@@ -654,7 +654,7 @@ def analyse_ntspectrum(self):
                     Z,
                     ion_stage,
                     ionpot_ev=ionpot_valence,
-                    Zbar=get_Zbar(ions=self.ionpopdict.keys(), ionpopdict=self.ionpopdict),
+                    Zbar=get_Zbar(ions=tuple(self.ionpopdict.keys()), ionpopdict=self.ionpopdict),
                 )
                 print(f"          eff_ionpot: {eff_ionpot:.2f} [eV]")
                 print(f"              workfn: {workfn_ev:.2f} [eV]")