diff --git a/doc/source/whatsnew/v1.5.0.rst b/doc/source/whatsnew/v1.5.0.rst
index 9a6455d4d012f..0b6e8ac7886d3 100644
--- a/doc/source/whatsnew/v1.5.0.rst
+++ b/doc/source/whatsnew/v1.5.0.rst
@@ -113,7 +113,7 @@ Bug fixes
 
 Categorical
 ^^^^^^^^^^^
--
+- Bug in :meth:`CategoricalIndex.union` when the index's categories are integer-dtype and the index contains ``NaN`` values incorrectly raising instead of casting to ``float64`` (:issue:`45362`)
 -
 
 Datetimelike
diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 3ea716d93aca0..9a85e2f8a5c1e 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -81,6 +81,7 @@
 )
 from pandas.core.dtypes.generic import (
     ABCExtensionArray,
+    ABCIndex,
     ABCSeries,
 )
 from pandas.core.dtypes.inference import is_list_like
@@ -93,7 +94,9 @@
 
 if TYPE_CHECKING:
 
+    from pandas import Index
     from pandas.core.arrays import (
+        Categorical,
         DatetimeArray,
         ExtensionArray,
         IntervalArray,
@@ -1470,6 +1473,44 @@ def find_result_type(left: ArrayLike, right: Any) -> DtypeObj:
     return new_dtype
 
 
+def common_dtype_categorical_compat(
+    objs: list[Index | ArrayLike], dtype: DtypeObj
+) -> DtypeObj:
+    """
+    Update the result of find_common_type to account for NAs in a Categorical.
+
+    Parameters
+    ----------
+    objs : list[np.ndarray | ExtensionArray | Index]
+    dtype : np.dtype or ExtensionDtype
+
+    Returns
+    -------
+    np.dtype or ExtensionDtype
+    """
+    # GH#38240
+
+    # TODO: more generally, could do `not can_hold_na(dtype)`
+    if isinstance(dtype, np.dtype) and dtype.kind in ["i", "u"]:
+
+        for obj in objs:
+            # We don't want to accientally allow e.g. "categorical" str here
+            obj_dtype = getattr(obj, "dtype", None)
+            if isinstance(obj_dtype, CategoricalDtype):
+                if isinstance(obj, ABCIndex):
+                    # This check may already be cached
+                    hasnas = obj.hasnans
+                else:
+                    # Categorical
+                    hasnas = cast("Categorical", obj)._hasnans
+
+                if hasnas:
+                    # see test_union_int_categorical_with_nan
+                    dtype = np.dtype(np.float64)
+                    break
+    return dtype
+
+
 @overload
 def find_common_type(types: list[np.dtype]) -> np.dtype:
     ...
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 95d9ed7adc360..241e0fea221d0 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -66,6 +66,7 @@
 
 from pandas.core.dtypes.cast import (
     can_hold_element,
+    common_dtype_categorical_compat,
     find_common_type,
     infer_dtype_from,
     maybe_cast_pointwise_result,
@@ -5976,17 +5977,8 @@ def _find_common_type_compat(self, target) -> DtypeObj:
                 return _dtype_obj
 
         dtype = find_common_type([self.dtype, target_dtype])
+        dtype = common_dtype_categorical_compat([self, target], dtype)
 
-        if dtype.kind in ["i", "u"]:
-            # TODO: what about reversed with self being categorical?
-            if (
-                isinstance(target, Index)
-                and is_categorical_dtype(target.dtype)
-                and target.hasnans
-            ):
-                # FIXME: find_common_type incorrect with Categorical GH#38240
-                # FIXME: some cases where float64 cast can be lossy?
-                dtype = np.dtype(np.float64)
         if dtype.kind == "c":
             dtype = _dtype_obj
         return dtype
diff --git a/pandas/tests/indexes/test_setops.py b/pandas/tests/indexes/test_setops.py
index a73ac89994761..a150a1f6d9494 100644
--- a/pandas/tests/indexes/test_setops.py
+++ b/pandas/tests/indexes/test_setops.py
@@ -592,6 +592,20 @@ def test_union_with_duplicate_index_not_subset_and_non_monotonic(cls):
     tm.assert_index_equal(result, expected)
 
 
+def test_union_int_categorical_with_nan():
+    ci = CategoricalIndex([1, 2, np.nan])
+    assert ci.categories.dtype.kind == "i"
+
+    idx = Index([1, 2])
+
+    result = idx.union(ci)
+    expected = Index([1, 2, np.nan], dtype=np.float64)
+    tm.assert_index_equal(result, expected)
+
+    result = ci.union(idx)
+    tm.assert_index_equal(result, expected)
+
+
 class TestSetOpsUnsorted:
     # These may eventually belong in a dtype-specific test_setops, or
     #  parametrized over a more general fixture