diff --git a/webbpsf/gridded_library.py b/webbpsf/gridded_library.py
index d1b95040..48ed4087 100644
--- a/webbpsf/gridded_library.py
+++ b/webbpsf/gridded_library.py
@@ -222,7 +222,13 @@ def _set_psf_locations(self, num_psfs, psf_location, psf_location_list):
                 location_list = psf_location_list
 
             else:
-                max_size = self.webb._detector_npixels - 1
+                if isinstance(self.webb._detector_npixels, tuple):
+                    max_size = min(self.webb._detector_npixels) - 1   # a tuple has been provided for non-square detector.
+                                                                      # This is only true for MIRI (1024, 1032) and it's a
+                                                                      # small enough effect that it's reasonable to treat
+                                                                      # it as square when choosing locatins here.
+                else:
+                    max_size = self.webb._detector_npixels - 1
                 loc_list = [int(round(num * max_size)) for num in np.linspace(0, 1, self.length, endpoint=True)]
                 location_list = list(itertools.product(loc_list, loc_list))  # list of tuples (x,y) (for WebbPSF)
 
diff --git a/webbpsf/tests/test_miri.py b/webbpsf/tests/test_miri.py
index 012dd5fb..0555544d 100644
--- a/webbpsf/tests/test_miri.py
+++ b/webbpsf/tests/test_miri.py
@@ -87,3 +87,10 @@ def test_miri_slit_apertures():
 
     assert np.isclose(miri._tel_coords()[0].to_value(u.arcsec), ap.V2Ref)
     assert np.isclose(miri._tel_coords()[1].to_value(u.arcsec), ap.V3Ref)
+
+def test_miri_nonsquare_detector():
+    """ Test that we can handle the slightly different
+    dimenssions in X and Y of the MIRI detector"""
+    miri = webbpsf_core.MIRI()
+    miri.detector_position = (1023, 1031)  # recall this is X, Y order
+    assert miri.detector_position == (1023, 1031)
diff --git a/webbpsf/webbpsf_core.py b/webbpsf/webbpsf_core.py
index c69a14a9..b05f3979 100644
--- a/webbpsf/webbpsf_core.py
+++ b/webbpsf/webbpsf_core.py
@@ -303,9 +303,13 @@ def detector_position(self, position):
             raise ValueError("Detector pixel coordinates must be pairs of nonnegative numbers, not {}".format(position))
         if x < 0 or y < 0:
             raise ValueError("Detector pixel coordinates must be nonnegative integers")
-        if x > self._detector_npixels - 1 or y > self._detector_npixels - 1:
-            raise ValueError("The maximum allowed detector pixel coordinate value is {}".format(
-                self._detector_npixels - 1))
+        if isinstance(self._detector_npixels, tuple):
+            det_npix_y, det_npix_x = self._detector_npixels  # A tuple has been provided for a non-square detector with different Y and X dimensions
+        else:
+            det_npix_y = det_npix_x = self._detector_npixels  # same dimensions in both X and Y
+
+        if x > det_npix_x- 1 or y > det_npix_y - 1:
+            raise ValueError(f"The maximum allowed detector pixel coordinate value is (X,Y) = ({det_npix_x-1}, {det_npix_y-1})")
 
         self._detector_position = (int(position[0]), int(position[1]))
 
@@ -700,7 +704,11 @@ def psf_grid(self, num_psfs=16, all_detectors=True, save=False,
             detectors = self.detector
 
         if single_psf_centered is True:
-            psf_location = (int((self._detector_npixels - 1) / 2), int((self._detector_npixels - 1) / 2))  # center pt
+            if isinstance(self._detector_npixels, tuple):
+                det_npix_y, det_npix_x = self._detector_npixels  # A tuple has been provided for a non-square detector with different Y and X dimensions
+            else:
+                det_npix_y = det_npix_x = self._detector_npixels  # same dimensions in both X and Y
+            psf_location = ( int(det_npix_x - 1) // 2, int(det_npix_y - 1) // 2)  # center pt
         else:
             psf_location = self.detector_position[::-1]  # (y,x)
 
@@ -1835,7 +1843,7 @@ def __init__(self):
 
         self._detectors = {'MIRIM': 'MIRIM_FULL'}  # Mapping from user-facing detector names to SIAF entries.
         self.detector = self.detector_list[0]
-        self._detector_npixels = 1024
+        self._detector_npixels = (1032, 1024)  # MIRI detector is not square
         self.detector_position = (512, 512)
 
         self._si_wfe_class = optics.MIRIFieldDependentAberrationAndObscuration