Based on the issue discussion and the hints provided, I can generate the fix. The problem is clearly identified in the GitHub issue discussion:

The `_entry` method of the `Identity` class in `sympy/matrices/expressions/matexpr.py` currently returns `S.One` when `i == j` and `S.Zero` otherwise. However, this comparison fails for symbolic indices because it incorrectly assumes that if `i` and `j` are different symbols, they represent different values.

The fix, as discussed in the issue, is to use `KroneckerDelta(i, j)` to properly handle the case where we don't know if `i == j` until actual values are substituted.

Here is the patch:

--- a/sympy/matrices/expressions/matexpr.py
+++ b/sympy/matrices/expressions/matexpr.py
@@ -1,6 +1,7 @@
 from __future__ import print_function, division
 
 from functools import wraps
+from sympy.functions.special.tensor_functions import KroneckerDelta
 
 from sympy.core import S, Symbol, Tuple, Integer, Basic, Expr
 from sympy.core.decorators import call_highest_priority
@@ -278,10 +279,11 @@ class Identity(MatrixExpr):
         return self.rows
 
     def _entry(self, i, j):
-        if i == j:
-            return S.One
-        else:
-            return S.Zero
+        eq = Eq(i, j)
+        if eq is S.true:
+            return S.One
+        elif eq is S.false:
+            return S.Zero
+        return KroneckerDelta(i, j)
 
     def _eval_transpose(self):
         return self