diff --git a/pycontrails/datalib/ecmwf/era5.py b/pycontrails/datalib/ecmwf/era5.py
index 5ed9eea3..3a00ab84 100644
--- a/pycontrails/datalib/ecmwf/era5.py
+++ b/pycontrails/datalib/ecmwf/era5.py
@@ -541,7 +541,7 @@ def _preprocess_era5_dataset(self, ds: xr.Dataset) -> xr.Dataset:
 
         # For "reanalysis-era5-single-levels"
         # then the netcdf file does not contain the dimension "level"
-        if "single-levels" in self.dataset:
+        if self.is_single_level:
             ds = ds.expand_dims(level=self.pressure_levels)
 
         # New CDS-Beta gives "valid_time" instead of "time"