diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index adee0ca..9480640 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,7 +21,7 @@ jobs:
         # and OS versions as possible (especially windows to create .whl packages,
         # see step "Create wheel and dist package")
         python-version: ['3.9', '3.10', '3.11', '3.12']
-        os: ["ubuntu-latest", "windows-latest", "macos-latest"]
+        os: ["ubuntu-22.04", "windows-2022", "macos-14"]
     name: py${{ matrix.python-version }}@${{ matrix.os }}
     runs-on: ${{ matrix.os }}
 
@@ -83,7 +83,7 @@ jobs:
           elif [ "${{ runner.os }}" == "Windows" ]; then
             OS="win"
           fi
-          
+
           pip install setuptools_scm twine cibuildwheel
           python setup.py sdist --dist-dir artifacts/dist_whl
           export CIBW_BUILD="cp${PY}-${OS}*"
diff --git a/src/pytesmo/interpolate/dctpls.py b/src/pytesmo/interpolate/dctpls.py
index ac21b08..0823362 100644
--- a/src/pytesmo/interpolate/dctpls.py
+++ b/src/pytesmo/interpolate/dctpls.py
@@ -38,12 +38,6 @@ class ValueRange:
     max: float
 
 
-exit_flags = {
-    1: 'OK',
-    2: 'less than 2 data samples',
-    3: 'Inner loop did not converge',
-}
-
 logger = pytesmolog
 streamHandler = logging.StreamHandler(sys.stdout)
 formatter = logging.Formatter(
@@ -359,7 +353,7 @@ def smoothn(
         axis: Optional[Union[int, Tuple[int, ...]]] = None,
         data_weights: Optional[np.ndarray] = None,
         smoothOrder: Optional[int] = 2,
-        init_guess=None,
+        init_guess: Optional[np.ndarray] = None,
         isrobust: Optional[bool] = True,
         MaxIter: Optional[int] = 100,
         TolZ: Annotated[float, ValueRange(0.0, 1.0)] = 0.001,