diff --git a/bilby/gw/psi4_waveform_generator.py b/bilby/gw/psi4_waveform_generator.py
index 45f3455c13f5b1cb8f30e456ec5eb1c2263d2509..1b754e211941c1da8daa9548b8cb861208c73d90 100644
--- a/bilby/gw/psi4_waveform_generator.py
+++ b/bilby/gw/psi4_waveform_generator.py
@@ -130,6 +130,8 @@ class Psi4_WaveformGenerator(WaveformGenerator):
             FD_psi4 = self._FD_psi4_from_FD_waveform(model_waveform) \
                     if is_FD_model else \
                     self._FD_psi4_from_TD_waveform(model_waveform)
+        elif model_waveform is None:
+            FD_psi4 = None
         else:
             FD_psi4 = dict()
             for key in model_waveform:
@@ -154,6 +156,8 @@ class Psi4_WaveformGenerator(WaveformGenerator):
 
         if isinstance(FD_psi4, np.ndarray):
             TD_psi4 = utils.infft(FD_psi4, self.sampling_frequency)
+        elif FD_psi4 is None:
+            TD_psi4 = None
         else:
             TD_psi4 = dict()
             for key in FD_psi4: