diff --git a/astropy/wcs/tests/test_utils.py b/astropy/wcs/tests/test_utils.py
index 36a851dcaea..ead7aac6cb2 100644
--- a/astropy/wcs/tests/test_utils.py
+++ b/astropy/wcs/tests/test_utils.py
@@ -29,6 +29,8 @@
 from astropy.utils.exceptions import AstropyUserWarning
 from astropy.wcs import _wcs
 from astropy.wcs.utils import (
+    FRAME_WCS_MAPPINGS,
+    WCS_FRAME_MAPPINGS,
     _pixel_to_pixel_correlation_matrix,
     _pixel_to_world_correlation_matrix,
     _split_matrix,
@@ -417,7 +419,10 @@ def test_wcs_to_body_frame():
 
     unknown_wcs = WCS(naxis=2)
     unknown_wcs.wcs.ctype = ["UTLN-TAN", "UTLT-TAN"]
-    with pytest.raises(KeyError, match="unknown solar system object abbreviation UT"):
+    with pytest.raises(
+        ValueError,
+        match="Could not determine celestial frame corresponding to the specified WCS object",
+    ):
         frame = wcs_to_celestial_frame(unknown_wcs)
 
     triaxial_wcs = WCS(naxis=2)
@@ -1593,3 +1598,40 @@ def test_obsgeo_infinite(dkist_location):
 def test_obsgeo_invalid(obsgeo):
     with pytest.raises(ValueError):
         obsgeo_to_frame(obsgeo, None)
+
+
+def test_custom_wcs_to_from_frame():
+    # See https://github.com/astropy/astropy/issues/15625
+    # test from Sam van Kooten
+
+    class CustomFrame(BaseCoordinateFrame):
+        obstime = Time("2017-08-17T12:41:04.43")
+
+    def custom_wcs_frame_mapping(wcs):
+        ctypes = {c[:4] for c in wcs.wcs.ctype}
+        if not ({"CSLN", "CSLT"} <= ctypes):
+            return None
+
+        dateobs = wcs.wcs.dateavg or wcs.wcs.dateobs or None
+        custom_frame = CustomFrame()
+        return custom_frame
+
+    def custom_frame_wcs_mapping(frame, projection="TAN"):
+        if not isinstance(frame, CustomFrame):
+            return None
+        wcs = WCS(naxis=2)
+        wcs.wcs.ctype = [f"CSLN-{projection}", f"CSLT-{projection}"]
+        return wcs
+
+    WCS_FRAME_MAPPINGS.append([custom_wcs_frame_mapping])
+    FRAME_WCS_MAPPINGS.append([custom_frame_wcs_mapping])
+
+    mywcs = WCS(naxis=2)
+    mywcs.wcs.ctype = ["CSLN-TAN", "CSLT-TAN"]
+    custom_frame = custom_wcs_frame_mapping(mywcs)
+    assert isinstance(custom_frame, CustomFrame)
+
+    custom_wcs = custom_frame_wcs_mapping(custom_frame)
+    print(custom_wcs.wcs.ctype)
+    assert custom_wcs.wcs.ctype[0] == "CSLN-TAN"
+    assert custom_wcs.wcs.ctype[1] == "CSLT-TAN"
diff --git a/astropy/wcs/utils.py b/astropy/wcs/utils.py
index a7149fe16a5..ed98fc4e41a 100644
--- a/astropy/wcs/utils.py
+++ b/astropy/wcs/utils.py
@@ -158,13 +158,11 @@ def _wcs_to_celestial_frame_builtin(wcs):
                 representation_type=SphericalRepresentation,
                 obstime=wcs.wcs.dateobs or None,
             )
-        elif xcoord[2:4] in ("LN", "LT") and "H" not in xcoord and "CR" not in xcoord:
+        elif xcoord[2:4] in ("LN", "LT") and xcoord[:2] in SOLAR_SYSTEM_OBJ_DICT.keys():
             # Coordinates on a planetary body, as defined in
             # https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018EA000388
 
             object_name = SOLAR_SYSTEM_OBJ_DICT.get(xcoord[:2])
-            if object_name is None:
-                raise KeyError(f"unknown solar system object abbreviation {xcoord[:2]}")
 
             a_radius = wcs.wcs.aux.a_radius
             b_radius = wcs.wcs.aux.b_radius
diff --git a/docs/changes/wcs/15630.bugfix.rst b/docs/changes/wcs/15630.bugfix.rst
new file mode 100644
index 00000000000..35ff04bd494
--- /dev/null
+++ b/docs/changes/wcs/15630.bugfix.rst
@@ -0,0 +1,2 @@
+Fix a regression in custom WCS mapping due to the recent introduction of
+Solar System frames.