diff --git a/kafe2/fit/_base/model.py b/kafe2/fit/_base/model.py
index e9d13e85..665ea2ea 100644
--- a/kafe2/fit/_base/model.py
+++ b/kafe2/fit/_base/model.py
@@ -215,12 +215,7 @@ def parcount(self):
     @property
     def x_name(self):
         """The name of the independent variable. ``None`` for 0 independent variables."""
-        # TODO: don't use case differentiation to maintain compatibility with current fit objects
-        if self._independent_argcount == 0:
-            return None
         _pars = list(self.signature.parameters.keys())
-        if self._independent_argcount == 1:
-            return _pars[0]
         return _pars[0 : self._independent_argcount]
 
     @property
@@ -262,12 +257,8 @@ def defaults(self, new_defaults):
     def defaults_dict(self):
         """The default values for model function parameters as a dict"""
         _defaults_dict = OrderedDict()
-        _x_name = self.x_name
+        _x_name = self.x_name  # list of strings
 
-        # _x_name can be a list for >1 independent variables.
-        # Always convert to list for simpler logic.
-        if type(_x_name) is str:
-            _x_name = [_x_name]
         for _par in self.signature.parameters.values():
             # skip independent variable parameter
             if _x_name is not None and _par.name in _x_name:
diff --git a/kafe2/test/fit/test_model_functions.py b/kafe2/test/fit/test_model_functions.py
index 006cd793..cb28b444 100644
--- a/kafe2/test/fit/test_model_functions.py
+++ b/kafe2/test/fit/test_model_functions.py
@@ -26,7 +26,7 @@ def model(x, a, b, c):
             parameter_names=["a", "b", "c"],
             argcount=4,
             parcount=3,
-            x_name="x",
+            x_name=["x"],
             defaults=[1.0, 1.0, 1.0],
             latex_name=r"{\tt model}",
             parameter_latex_names=["{a}", "{b}", "{c}"],
@@ -48,7 +48,7 @@ def other_model(tau, A_0=3.0, tau_0=1.0):
             parameter_names=["A_0", "tau_0"],
             argcount=3,
             parcount=2,
-            x_name="tau",
+            x_name=["tau"],
             defaults=[3.0, 1.0],
             latex_name=r"{\tt other\_model}",
             parameter_latex_names=["{A_0}", r"{\tau_0}"],
@@ -118,14 +118,16 @@ class TestSymPyIndexed(TestSymPy):
 
     def setUp(self):
         super().setUp()
-        self._ref_properties_1["parameter_names"].insert(0, self._ref_properties_1["x_name"])
+        self._ref_properties_1["parameter_names"] = \
+            self._ref_properties_1["x_name"] + self._ref_properties_1["parameter_names"]
         self._ref_properties_1["parcount"] += 1
-        self._ref_properties_1["x_name"] = None
+        self._ref_properties_1["x_name"] = []
         self._ref_properties_1["defaults"].insert(0, 1.0)
         self._ref_properties_1["parameter_latex_names"].insert(0, r"{x}")
 
-        self._ref_properties_2["parameter_names"].insert(0, self._ref_properties_2["x_name"])
+        self._ref_properties_2["parameter_names"] = \
+            self._ref_properties_2["x_name"] + self._ref_properties_2["parameter_names"]
         self._ref_properties_2["parcount"] += 1
-        self._ref_properties_2["x_name"] = None
+        self._ref_properties_2["x_name"] = []
         self._ref_properties_2["defaults"].insert(0, 1.0)
         self._ref_properties_2["parameter_latex_names"].insert(0, r"{\tau}")