diff --git a/weatherbench2/derived_variables.py b/weatherbench2/derived_variables.py
index 774299c..09a22d2 100644
--- a/weatherbench2/derived_variables.py
+++ b/weatherbench2/derived_variables.py
@@ -664,7 +664,8 @@ def interpolate_spectral_frequencies(
 
   def interp_at_one_lat(da: xr.DataArray) -> xr.DataArray:
     da = (
-        da.swap_dims({wavenumber_dim: 'frequency'})  # pytype: disable=wrong-arg-types
+        da.squeeze('latitude')
+        .swap_dims({wavenumber_dim: 'frequency'})  # pytype: disable=wrong-arg-types
         .drop_vars(wavenumber_dim)
         .interp(frequency=frequencies, method=method, **interp_kwargs)
     )
@@ -673,7 +674,7 @@ def interp_at_one_lat(da: xr.DataArray) -> xr.DataArray:
     da['wavelength'] = da['wavelength'].assign_attrs(units='m')
     return da
 
-  return spectrum.groupby('latitude').apply(interp_at_one_lat)
+  return spectrum.groupby('latitude', squeeze=False).apply(interp_at_one_lat)
 
 
 @dataclasses.dataclass