diff --git a/OMPython/ModelicaSystem.py b/OMPython/ModelicaSystem.py
index e362520a..ee444c0f 100644
--- a/OMPython/ModelicaSystem.py
+++ b/OMPython/ModelicaSystem.py
@@ -314,7 +314,7 @@ def parse_simflags(simflags: str) -> dict[str, Optional[str | dict[str, str]]]:
 class ModelicaSystem:
     def __init__(
             self,
-            fileName: Optional[str | os.PathLike] = None,
+            fileName: Optional[str | os.PathLike | pathlib.Path] = None,
             modelName: Optional[str] = None,
             lmodel: Optional[list[str | tuple[str, str]]] = None,
             commandLineOptions: Optional[str] = None,
@@ -360,9 +360,13 @@ def __init__(
             mod = ModelicaSystem("ModelicaModel.mo", "modelName", ["Modelica"])
             mod = ModelicaSystem("ModelicaModel.mo", "modelName", [("Modelica","3.2.3"), "PowerSystems"])
         """
+
         if fileName is None and modelName is None and not lmodel:  # all None
             raise ModelicaSystemError("Cannot create ModelicaSystem object without any arguments")
 
+        if modelName is None:
+            raise ModelicaSystemError("A modelname must be provided (argument modelName)!")
+
         self.quantitiesList = []
         self.paramlist = {}
         self.inputlist = {}
diff --git a/tests/test_ModelicaSystem.py b/tests/test_ModelicaSystem.py
index 202e066d..71da85c8 100644
--- a/tests/test_ModelicaSystem.py
+++ b/tests/test_ModelicaSystem.py
@@ -57,7 +57,7 @@ def test_setParameters():
 def test_setSimulationOptions():
     omc = OMPython.OMCSessionZMQ()
     model_path = omc.sendExpression("getInstallationDirectoryPath()") + "/share/doc/omc/testmodels/"
-    mod = OMPython.ModelicaSystem(model_path + "BouncingBall.mo", "BouncingBall")
+    mod = OMPython.ModelicaSystem(fileName=model_path + "BouncingBall.mo", modelName="BouncingBall")
 
     # method 1
     mod.setSimulationOptions("stopTime=1.234")
@@ -88,7 +88,7 @@ def test_relative_path(model_firstorder):
         model_relative = str(model_file)
         assert "/" not in model_relative
 
-        mod = OMPython.ModelicaSystem(model_relative, "M")
+        mod = OMPython.ModelicaSystem(fileName=model_relative, modelName="M")
         assert float(mod.getParameters("a")[0]) == -1
     finally:
         model_file.unlink()  # clean up the temporary file
@@ -145,7 +145,7 @@ def test_getters(tmp_path):
 y = der(x);
 end M_getters;
 """)
-    mod = OMPython.ModelicaSystem(model_file.as_posix(), "M_getters")
+    mod = OMPython.ModelicaSystem(fileName=model_file.as_posix(), modelName="M_getters")
 
     q = mod.getQuantities()
     assert isinstance(q, list)
@@ -324,7 +324,7 @@ def test_simulate_inputs(tmp_path):
 y = x;
 end M_input;
 """)
-    mod = OMPython.ModelicaSystem(model_file.as_posix(), "M_input")
+    mod = OMPython.ModelicaSystem(fileName=model_file.as_posix(), modelName="M_input")
 
     mod.setSimulationOptions("stopTime=1.0")
 
diff --git a/tests/test_ModelicaSystemCmd.py b/tests/test_ModelicaSystemCmd.py
index f82510df..32f111b2 100644
--- a/tests/test_ModelicaSystemCmd.py
+++ b/tests/test_ModelicaSystemCmd.py
@@ -16,7 +16,7 @@ def model_firstorder(tmp_path):
 
 
 def test_simflags(model_firstorder):
-    mod = OMPython.ModelicaSystem(model_firstorder.as_posix(), "M")
+    mod = OMPython.ModelicaSystem(fileName=model_firstorder.as_posix(), modelName="M")
     mscmd = OMPython.ModelicaSystemCmd(runpath=mod.tempdir, modelname=mod.modelName)
     mscmd.args_set({
         "noEventEmit": None,
diff --git a/tests/test_linearization.py b/tests/test_linearization.py
index e9f0f6d7..2c79190c 100644
--- a/tests/test_linearization.py
+++ b/tests/test_linearization.py
@@ -55,7 +55,7 @@ def test_getters(tmp_path):
 y2 = phi + u1;
 end Pendulum;
 """)
-    mod = OMPython.ModelicaSystem(model_file.as_posix(), "Pendulum", ["Modelica"])
+    mod = OMPython.ModelicaSystem(fileName=model_file.as_posix(), modelName="Pendulum", lmodel=["Modelica"])
 
     d = mod.getLinearizationOptions()
     assert isinstance(d, dict)
diff --git a/tests/test_optimization.py b/tests/test_optimization.py
index 672de4a6..aa74df79 100644
--- a/tests/test_optimization.py
+++ b/tests/test_optimization.py
@@ -33,7 +33,7 @@ def test_optimization_example(tmp_path):
 end BangBang2021;
 """)
 
-    mod = OMPython.ModelicaSystem(model_file.as_posix(), "BangBang2021")
+    mod = OMPython.ModelicaSystem(fileName=model_file.as_posix(), modelName="BangBang2021")
 
     mod.setOptimizationOptions(["numberOfIntervals=16", "stopTime=1",
                                 "stepSize=0.001", "tolerance=1e-8"])