diff --git a/src/qibolab/kernels.py b/src/qibolab/kernels.py
index 480fe7841..654f97575 100644
--- a/src/qibolab/kernels.py
+++ b/src/qibolab/kernels.py
@@ -29,4 +29,6 @@ def dump(self, path: Path):
         The keys (QubitId) are serialized to strings and the values
         (numpy arrays) are kept as is.
         """
-        np.savez(path, **{json.dumps(key): value for key, value in self.items()})
+        np.savez(
+            path, **{json.dumps(qubit_id): value for qubit_id, value in self.items()}
+        )
diff --git a/src/qibolab/serialize.py b/src/qibolab/serialize.py
index d4c971e1a..65cdc70a6 100644
--- a/src/qibolab/serialize.py
+++ b/src/qibolab/serialize.py
@@ -23,6 +23,8 @@
 )
 from qibolab.qubits import Qubit, QubitPair
 
+KERNELS_FILE = "kernels.npz"
+
 
 def load_runcard(path: Path) -> dict:
     """Load runcard YAML to a dictionary."""
@@ -51,7 +53,7 @@ def load_qubits(
     }
 
     if extras_folder is not None:
-        kernels = Kernels.load(path=extras_folder / "kernels.npz")
+        kernels = Kernels.load(path=extras_folder / KERNELS_FILE)
         for q in kernels:
             qubits[q].kernel = kernels[q]
 
@@ -178,12 +180,12 @@ def dump_runcard(platform: Platform, path: Path):
         path (pathlib.Path): Path that the yaml file will be saved.
     """
 
-    kernels = {}
+    kernels = Kernels()
     for qubit in platform.qubits.values():
         if qubit.kernel is not None:
-            kernels[str(qubit.name)] = qubit.kernel
+            kernels[qubit.name] = qubit.kernel
             qubit.kernel = None
-    Kernels(kernels).dump(Path(__file__).parent / "dummy/kernels.npz")
+    Kernels(kernels).dump(Path(__file__).parent / "dummy" / KERNELS_FILE)
 
     settings = {
         "nqubits": platform.nqubits,
diff --git a/tests/dummy_qrc/zurich.py b/tests/dummy_qrc/zurich.py
index e1a6d8d32..1ff4fe654 100644
--- a/tests/dummy_qrc/zurich.py
+++ b/tests/dummy_qrc/zurich.py
@@ -171,8 +171,8 @@ def create(runcard_path=RUNCARD):
         channels[ch].local_oscillator = local_oscillators[lo]
 
     # create qubit objects
-    runcard = load_runcard(runcard_path, FOLDER)
-    qubits, couplers, pairs = load_qubits(runcard)
+    runcard = load_runcard(runcard_path)
+    qubits, couplers, pairs = load_qubits(runcard, FOLDER)
     settings = load_settings(runcard)
 
     # assign channels to qubits and sweetspots(operating points)