fix by performing check

xarray/core/coordinates.py

@@ -22,7 +22,11 @@
     assert_no_index_corrupted,
     create_default_index_implicit,
 )
-from xarray.core.merge import merge_coordinates_without_align, merge_coords
+from xarray.core.merge import (
+    assert_valid_explicit_coords,
+    merge_coordinates_without_align,
+    merge_coords,
+)
 from xarray.core.types import DataVars, Self, T_DataArray, T_Xarray
 from xarray.core.utils import (
     Frozen,
@@ -306,6 +310,14 @@ def __init__(
                 else:
                     variables[name] = var
 
+            dims = set(d for var in variables.values() for d in var.dims)
+            sizes = {
+                d: -1 for d in dims
+            }  # TODO the lengths here are never used, so assert_valid_explicit_coords should be refactored
+            assert_valid_explicit_coords(
+                variables, dims=sizes, explicit_coords=list(variables)
+            )
+
         if indexes is None:
             indexes = {}
         else: