diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f74a9ae..0713437 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -15,69 +15,69 @@ jobs:
 
         steps:
             # Checkout the code
-          - name: Checkout code
-            uses: actions/checkout@v4
+            - name: Checkout code
+              uses: actions/checkout@v4
 
-          - uses: mamba-org/setup-micromamba@v1
-            with:
-                micromamba-version: "1.5.9-1" # any version from https://github.com/mamba-org/micromamba-releases
-                channels: tcevaer, conda-forge
-                init-shell: bash
-                post-cleanup: "all"
+            - uses: mamba-org/setup-micromamba@v1
+              with:
+                  micromamba-version: "1.5.9-1" # any version from https://github.com/mamba-org/micromamba-releases
+                  channels: tcevaer, conda-forge
+                  init-shell: bash
+                  post-cleanup: "all"
 
-          - name: Configure Conda channel priority to disabled
-            run: |
-                conda config --set channel_priority disabled
+            - name: Configure Conda channel priority to disabled
+              run: |
+                  conda config --set channel_priority disabled
 
-          - name: Create environment and install tools
-            run: micromamba create -n grdwind_env pytest conda-build boa python=3.10 -y
+            - name: Create environment and install tools
+              run: micromamba create -n grdwind_env pytest conda-build boa python=3.10 -y
 
-          - name: Build package
-            run: |
-                cd recipe
-                eval "$(micromamba shell hook --shell bash)"
-                micromamba activate grdwind_env
-                conda mambabuild .
+            - name: Build package
+              run: |
+                  cd recipe
+                  eval "$(micromamba shell hook --shell bash)"
+                  micromamba activate grdwind_env
+                  conda mambabuild .
 
-          # Install the built package into the environment
-          - name: Install the built package
-            run: |
-                eval "$(micromamba shell hook --shell bash)"
-                micromamba activate grdwind_env
-                conda install --use-local grdwindinversion -y
+            # Install the built package into the environment
+            - name: Install the built package
+              run: |
+                  eval "$(micromamba shell hook --shell bash)"
+                  micromamba activate grdwind_env
+                  conda install --use-local grdwindinversion -y
 
-            # Cache the test data if previously downloaded (up to 10 GB limit for the cache)
-            # WARNING : modify the key if the data is modified !!
-          - name: Cache test data
-            uses: actions/cache@v4
-            id: cache
-            with:
-                path: ./test_data
-                key: test-data-v3
-                restore-keys: test-data-v3
+              # Cache the test data if previously downloaded (up to 10 GB limit for the cache)
+              # WARNING : modify the key if the data is modified !!
+            - name: Cache test data
+              uses: actions/cache@v4
+              id: cache
+              with:
+                  path: ./test_data
+                  key: test-data-v3
+                  restore-keys: test-data-v3
 
-            # Download test data if not already cached
-          - name: Download test data
-            if: steps.cache.outputs.cache-hit != 'true' # Only download if cache miss
-            run: |
+              # Download test data if not already cached
+            - name: Download test data
+              if: steps.cache.outputs.cache-hit != 'true' # Only download if cache miss
+              run: |
                   mkdir -p ./test_data/
                   wget https://cloud.ifremer.fr/index.php/s/ExLQ2TnYAqozPWE/download -O /tmp/ecmwf.zip
                   unzip /tmp/ecmwf.zip -d ./test_data/
                   wget https://cloud.ifremer.fr/index.php/s/kRgdOOPsjoZieZR/download -O /tmp/l1.zip
                   unzip /tmp/l1.zip -d ./test_data/
-            timeout-minutes: 200 # Adjust depending on the size of your data
+              timeout-minutes: 200 # Adjust depending on the size of your data
 
-            # Set up xsar configuration
-          - name: Setup xsar configuration
-            run: |
+              # Set up xsar configuration
+            - name: Setup xsar configuration
+              run: |
                   mkdir -p ~/.xsar
                   echo "data_dir: /tmp" > ~/.xsar/config.yaml
                   echo "auxiliary_dir: ./test_data/auxiliary" >> ~/.xsar/config.yaml
                   echo "path_dataframe_aux: ./test_data/auxiliary/active_aux.csv" >> ~/.xsar/config.yaml
 
-            # Set up grdwindinversion configuration
-          - name: Setup grdwindinversion configuration
-            run: |
+              # Set up grdwindinversion configuration
+            - name: Setup grdwindinversion configuration
+              run: |
                   mkdir -p ~/.grdwindinversion
                   echo "'ecmwf_0100_1h': ./test_data/ECMWF/forecast/hourly/0100deg/netcdf_light/%Y/%j/ECMWF_FORECAST_0100_%Y%m%d%H%M_10U_10V.nc" > ~/.grdwindinversion/data_config.yaml
                   echo "'ecmwf_0125_1h': ./test_data/ECMWF/0.125deg/1h/forecasts/%Y/%j/ecmwf_%Y%m%d%H%M.nc" >> ~/.grdwindinversion/data_config.yaml
@@ -85,9 +85,9 @@ jobs:
                   #echo "'lut_cmod7_path': './test_data/GMFS/v1.6/GMF_cmod7_official/cmod7_and_python_script'" >> ~/.grdwindinversion/data_config.yaml
                   #echo "'lut_ms1ahw_path': './test_data/GMFS/v1.6/GMF_cmodms1ahw'" >> ~/.grdwindinversion/data_config.yaml
 
-            # Run the tests
-          - name: Run tests
-            run: |
+              # Run the tests
+            - name: Run tests
+              run: |
                   eval "$(micromamba shell hook --shell bash)"
                   micromamba activate grdwind_env
                   pytest
diff --git a/grdwindinversion/inversion.py b/grdwindinversion/inversion.py
index 3ffcabd..2006a43 100644
--- a/grdwindinversion/inversion.py
+++ b/grdwindinversion/inversion.py
@@ -797,7 +797,7 @@ def preprocess(filename, outdir, config_path, overwrite=False, add_gradientsfeat
             xr_dataset.sigma0.sel(pol=crosspol), xr_dataset.incidence, model=model_cross)
         if config["apply_flattening"]:
             xr_dataset = xr_dataset.assign(nesz_cross_flattened=(
-                ['line', 'sample'], windspeed.nesz_flattening(xr_dataset.nesz.sel(pol=crosspol), xr_dataset.incidence)))
+                ['line', 'sample'], windspeed.nesz_flattening(xr_dataset.nesz.sel(pol=crosspol), xr_dataset.incidence).data))
             xr_dataset['nesz_cross_flattened'].attrs[
                 "comment"] = 'nesz has been flattened using windspeed.nesz_flattening'
             # dsig