diff --git a/tupak/gw/waveform_generator.py b/tupak/gw/waveform_generator.py
index fe87dc8acd2cf8a54916b0296050a623b0848cea..8e4531d190ddf31897045a0fce972686c96a555e 100644
--- a/tupak/gw/waveform_generator.py
+++ b/tupak/gw/waveform_generator.py
@@ -1,6 +1,5 @@
 import inspect
-
-from tupak.core import utils
+import tupak
 import numpy as np
 
 
@@ -84,7 +83,7 @@ class WaveformGenerator(object):
         """
         return self._calculate_strain(model=self.frequency_domain_source_model,
                                       model_data_points=self.frequency_array,
-                                      transformation_function=utils.nfft,
+                                      transformation_function=tupak.core.utils.nfft,
                                       transformed_model=self.time_domain_source_model,
                                       transformed_model_data_points=self.time_array)
 
@@ -106,7 +105,7 @@ class WaveformGenerator(object):
         """
         return self._calculate_strain(model=self.time_domain_source_model,
                                       model_data_points=self.time_array,
-                                      transformation_function=utils.infft,
+                                      transformation_function=tupak.core.utils.infft,
                                       transformed_model=self.frequency_domain_source_model,
                                       transformed_model_data_points=self.frequency_array)