diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 1ff53bd6..0d4351ee 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -42,7 +42,7 @@ jobs:
         python-version: ["3.11"]
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6
       with:
diff --git a/emukit/core/optimization/multi_source_acquisition_optimizer.py b/emukit/core/optimization/multi_source_acquisition_optimizer.py
index 0b95a6ba..8bf19250 100644
--- a/emukit/core/optimization/multi_source_acquisition_optimizer.py
+++ b/emukit/core/optimization/multi_source_acquisition_optimizer.py
@@ -61,8 +61,8 @@ def optimize(self, acquisition: Acquisition, context: Optional[Context] = None)
                         the parameter name and the value is the value to fix the parameter to.
         :return: A tuple of (location of maximum, acquisition value at maximum)
         """
-        f_maxs = np.zeros((len(self.source_parameter.domain)))
         x_opts = []
+        f_maxs = []
 
         if context is None:
             context = dict()
@@ -75,7 +75,10 @@ def optimize(self, acquisition: Acquisition, context: Optional[Context] = None)
             # Fix the source using a dictionary, the key is the name of the parameter to fix and the value is the
             # value to which the parameter is fixed
             context[self.source_parameter.name] = self.source_parameter.domain[i]
-            x, f_maxs[i] = self.acquisition_optimizer.optimize(acquisition, context)
+            x, f = self.acquisition_optimizer.optimize(acquisition, context)
             x_opts.append(x)
+            f_maxs.append(f)
+
+        f_maxs = np.concatenate(f_maxs)
         best_source = np.argmax(f_maxs)
         return x_opts[best_source], np.max(f_maxs)
diff --git a/tests/emukit/core/test_multi_source_optimizer.py b/tests/emukit/core/test_multi_source_optimizer.py
index 99b4bba0..477b68a1 100644
--- a/tests/emukit/core/test_multi_source_optimizer.py
+++ b/tests/emukit/core/test_multi_source_optimizer.py
@@ -19,7 +19,7 @@
 @pytest.fixture
 def multi_source_optimizer():
     mock_acquisition_optimizer = mock.create_autospec(GradientAcquisitionOptimizer)
-    mock_acquisition_optimizer.optimize.return_value = (np.array([[0.0]]), None)
+    mock_acquisition_optimizer.optimize.return_value = (np.array([[0.0]]), np.array([[0.0]]))
     space = ParameterSpace([ContinuousParameter("x", 0, 1), InformationSourceParameter(2)])
     return MultiSourceAcquisitionOptimizer(mock_acquisition_optimizer, space)