diff --git a/lib/matplotlib/category.py b/lib/matplotlib/category.py
index 55af459fd575..9013460311cc 100644
--- a/lib/matplotlib/category.py
+++ b/lib/matplotlib/category.py
@@ -138,7 +138,12 @@ def __init__(self, units_mapping):
         self._units = units_mapping
 
     def __call__(self, x, pos=None):
-        return '' if pos is None else self.format_ticks([x])[0]
+        """
+        Return the category label string for tick val *x*.
+
+        The position *pos* is ignored.
+        """
+        return self.format_ticks([x])[0]
 
     def format_ticks(self, values):
         r_mapping = {v: self._text(k) for k, v in self._units.items()}
diff --git a/lib/matplotlib/tests/test_category.py b/lib/matplotlib/tests/test_category.py
index 609c7a966cd9..d7c8df8e21b5 100644
--- a/lib/matplotlib/tests/test_category.py
+++ b/lib/matplotlib/tests/test_category.py
@@ -154,15 +154,15 @@ def test_StrCategoryFormatter(self, ax, ydata):
         labels = cat.StrCategoryFormatter(unit._mapping)
         for i, d in enumerate(ydata):
             assert labels(i, i) == d
+            assert labels(i, None) == d
 
     @pytest.mark.parametrize("ydata", cases, ids=ids)
     @pytest.mark.parametrize("plotter", PLOT_LIST, ids=PLOT_IDS)
     def test_StrCategoryFormatterPlot(self, ax, ydata, plotter):
         plotter(ax, range(len(ydata)), ydata)
         for i, d in enumerate(ydata):
-            assert ax.yaxis.major.formatter(i, i) == d
-        assert ax.yaxis.major.formatter(i+1, i+1) == ""
-        assert ax.yaxis.major.formatter(0, None) == ""
+            assert ax.yaxis.major.formatter(i) == d
+        assert ax.yaxis.major.formatter(i+1) == ""
 
 
 def axis_test(axis, labels):