diff --git a/news/fix-get-array-index.rst b/news/fix-get-array-index.rst
new file mode 100644
index 0000000..67814f5
--- /dev/null
+++ b/news/fix-get-array-index.rst
@@ -0,0 +1,23 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* Let ``DiffractionObject.get_array_index`` to use the ``xtype`` from its inputs.
+
+**Security:**
+
+* <news item>
diff --git a/src/diffpy/utils/diffraction_objects.py b/src/diffpy/utils/diffraction_objects.py
index 25f32f4..ec09b41 100644
--- a/src/diffpy/utils/diffraction_objects.py
+++ b/src/diffpy/utils/diffraction_objects.py
@@ -409,17 +409,17 @@ def uuid(self):
     def uuid(self, _):
         raise AttributeError(_setter_wmsg("uuid"))
 
-    def get_array_index(self, xtype, xvalue):
-        """Return the index of the closest value in the array associated with
+    def get_array_index(self, xvalue, xtype=None):
+        f"""Return the index of the closest value in the array associated with
         the specified xtype and the value provided.
 
         Parameters
         ----------
-        xtype : str
-            The type of the independent variable in `xarray`. Must be one
-            of {*XQUANTITIES}.
         xvalue : float
             The value of the xtype to find the closest index for.
+        xtype : str, optional
+            The type of the independent variable in `xarray`. Must be one
+            of {*XQUANTITIES, }. Default is {self._input_xtype}
 
         Returns
         -------
@@ -427,8 +427,11 @@ def get_array_index(self, xtype, xvalue):
             The index of the closest value in the array associated with the
             specified xtype and the value provided.
         """
-
-        xtype = self._input_xtype
+        if xtype is None:
+            xtype = self._input_xtype
+        else:
+            if xtype not in XQUANTITIES:
+                raise ValueError(_xtype_wmsg(xtype))
         xarray = self.on_xtype(xtype)[0]
         if len(xarray) == 0:
             raise ValueError(
diff --git a/tests/test_diffraction_objects.py b/tests/test_diffraction_objects.py
index 8ed1dd3..59ee593 100644
--- a/tests/test_diffraction_objects.py
+++ b/tests/test_diffraction_objects.py
@@ -376,8 +376,8 @@ def test_scale_to_bad(org_do_args, target_do_args, scale_inputs):
             },
             0,
         ),
-        (  # C2: Target value lies in the array, expect the (first) closest
-            # index
+        # C2: Target value lies in the array, expect the closest index
+        (  # 1. xtype(tth) is equal to self._input_xtype(tth)
             {
                 "wavelength": 4 * np.pi,
                 "xarray": np.array([30, 60]),
@@ -390,7 +390,7 @@ def test_scale_to_bad(org_do_args, target_do_args, scale_inputs):
             },
             0,
         ),
-        (
+        (  # 2. use default xtype(equal to self._input_xtype)
             {
                 "wavelength": 4 * np.pi,
                 "xarray": np.array([30, 60]),
@@ -398,11 +398,24 @@ def test_scale_to_bad(org_do_args, target_do_args, scale_inputs):
                 "xtype": "tth",
             },
             {
-                "xtype": "q",
-                "value": 0.25,
+                "xtype": None,
+                "value": 45,
             },
             0,
         ),
+        (  # 3. xtype(q) is different from self._input_xtype(tth)
+            {
+                "wavelength": 4 * np.pi,
+                "xarray": np.array([30, 60]),
+                "yarray": np.array([1, 2]),
+                "xtype": "tth",
+            },
+            {
+                "xtype": "q",
+                "value": 0.5,
+            },
+            1,
+        ),
         # C3: Target value out of the range, expect the closest index
         (  # 1. Test with xtype of "q"
             {
@@ -435,12 +448,13 @@ def test_scale_to_bad(org_do_args, target_do_args, scale_inputs):
 def test_get_array_index(do_args, get_array_index_inputs, expected_index):
     do = DiffractionObject(**do_args)
     actual_index = do.get_array_index(
-        get_array_index_inputs["xtype"], get_array_index_inputs["value"]
+        get_array_index_inputs["value"], get_array_index_inputs["xtype"]
     )
     assert actual_index == expected_index
 
 
 def test_get_array_index_bad():
+    # empty array in DiffractionObject
     do = DiffractionObject(
         wavelength=2 * np.pi,
         xarray=np.array([]),
@@ -454,6 +468,22 @@ def test_get_array_index_bad():
         ),
     ):
         do.get_array_index(xtype="tth", xvalue=30)
+    # non-existing xtype
+    do = DiffractionObject(
+        wavelength=4 * np.pi,
+        xarray=np.array([30, 60]),
+        yarray=np.array([1, 2]),
+        xtype="tth",
+    )
+    non_existing_xtype = "non_existing_xtype"
+    with pytest.raises(
+        ValueError,
+        match=re.escape(
+            f"I don't know how to handle the xtype, '{non_existing_xtype}'. "
+            f"Please rerun specifying an xtype from {*XQUANTITIES, }"
+        ),
+    ):
+        do.get_array_index(xtype=non_existing_xtype, xvalue=30)
 
 
 def test_dump(tmp_path, mocker):