diff --git a/bilby/gw/waveform_generator.py b/bilby/gw/waveform_generator.py
index ba29af5b47eddfc5cfd705452ae9b61b9d1735d0..c5b42ec9129c80f0fc37cf5251a0b72f74cffb7a 100644
--- a/bilby/gw/waveform_generator.py
+++ b/bilby/gw/waveform_generator.py
@@ -167,7 +167,7 @@ class WaveformGenerator(object):
         model_strain = dict()
         for key in transformed_model_strain:
             if transformation_function == utils.nfft:
-                model_strain[key], = \
+                model_strain[key], _ = \
                     transformation_function(transformed_model_strain[key], self.sampling_frequency)
             else:
                 model_strain[key] = transformation_function(transformed_model_strain[key], self.sampling_frequency)