diff --git a/bilby/gw/source.py b/bilby/gw/source.py
index b8d5c2e3..bd2cd1b6 100644
--- a/bilby/gw/source.py
+++ b/bilby/gw/source.py
@@ -669,7 +669,7 @@ def _base_lal_cbc_fd_waveform(
         h_cross[frequency_bounds] *= time_shift
 
     if len(waveform_kwargs) > 0:
-        logger.warning(UNUSED_KWARGS_MESSAGE.format(waveform_kwargs))
+        logger.warning(UNUSED_KWARGS_MESSAGE.format(waveform_kwargs=waveform_kwargs))
 
     return dict(plus=h_plus, cross=h_cross)
 
@@ -1128,7 +1128,7 @@ def _base_waveform_frequency_sequence(
                 raise
 
     if len(waveform_kwargs) > 0:
-        logger.warning(UNUSED_KWARGS_MESSAGE.format(waveform_kwargs))
+        logger.warning(UNUSED_KWARGS_MESSAGE.format(waveform_kwargs=waveform_kwargs))
 
     return dict(plus=h_plus.data.data, cross=h_cross.data.data)