# sympy__sympy-17630
# Loki Mode Multi-Agent Patch
# Attempts: 1

The sympy repository is not available locally. Based on the issue description and the architect's analysis, I can generate the patch for the upstream SymPy repository. The bug is well-documented: when `ZeroMatrix * ZeroMatrix` or similar operations occur in `_blockmul`, the result is a scalar `Zero` instead of `ZeroMatrix`, causing subsequent operations to fail.

```
--- a/sympy/matrices/expressions/blockmatrix.py
+++ b/sympy/matrices/expressions/blockmatrix.py
@@ -1,5 +1,6 @@
 from __future__ import print_function, division
 
+from sympy import S
 from sympy.assumptions.ask import ask, Q
 from sympy.core import Basic, Add
 from sympy.strategies import typed, exhaust, condition, do_one, unpack
@@ -88,6 +89,23 @@ class BlockMatrix(MatrixExpr):
     def colblocksizes(self):
         return [self.blocks[0, i].cols for i in range(self.blockshape[1])]
 
+    def _ensure_zero_matrix(self, block, rows, cols):
+        """Convert scalar Zero to ZeroMatrix with proper dimensions."""
+        from sympy.matrices.expressions.matexpr import ZeroMatrix
+        if block == S.Zero:
+            return ZeroMatrix(rows, cols)
+        return block
+
+    def _get_block_sizes(self, blocks, rowblocksizes, colblocksizes):
+        """Get row and column sizes for each block position."""
+        result = []
+        for i in range(len(rowblocksizes)):
+            row = []
+            for j in range(len(colblocksizes)):
+                row.append((rowblocksizes[i], colblocksizes[j]))
+            result.append(row)
+        return result
+
     def _blockmul(self, other):
         if (isinstance(other, BlockMatrix) and
                 self.colblocksizes == other.rowblocksizes):
@@ -95,8 +113,14 @@ class BlockMatrix(MatrixExpr):
                         for i in range(self.blockshape[0])
                         for j in range(other.blockshape[1])]
 
-            return BlockMatrix(Matrix(self.blockshape[0], other.blockshape[1], block_mul))
-        return self * other
+            result = BlockMatrix(Matrix(self.blockshape[0], other.blockshape[1], block_mul))
+            # Convert any scalar Zero entries back to ZeroMatrix with proper dimensions
+            rowsizes = self.rowblocksizes
+            colsizes = other.colblocksizes
+            new_blocks = [[self._ensure_zero_matrix(result.blocks[i, j], rowsizes[i], colsizes[j])
+                          for j in range(result.blockshape[1])]
+                          for i in range(result.blockshape[0])]
+            return BlockMatrix(Matrix(new_blocks))
+        return self * other
 
     def _blockadd(self, other):
         if (isinstance(other, BlockMatrix)
```

Wait, I need to reconsider this approach. The issue is that when we compute `MatMul(A, ZeroMatrix)` or `MatMul(ZeroMatrix, ZeroMatrix)`, it gets simplified to scalar `Zero` rather than `ZeroMatrix`. The fix should handle this in the `_blockmul` method where the matrix multiplication results are computed.

Let me provide a cleaner patch:

```
--- a/sympy/matrices/expressions/blockmatrix.py
+++ b/sympy/matrices/expressions/blockmatrix.py
@@ -163,6 +163,8 @@ def bc_matmul(expr):
 def bc_block_plus_ident(expr):
     pass
 
+from sympy.matrices.expressions.matexpr import ZeroMatrix
+
 def _block_mul(A, B):
     """Multiply two block matrices.
     Assumes that both matrices have conforming block structures.
@@ -176,6 +178,10 @@ def _block_mul(A, B):
             block = Add(*[A.blocks[i, k] * B.blocks[k, j]
                         for k in range(A.blockshape[1])])
             blocks[i*n + j] = block
+            # Convert scalar Zero back to ZeroMatrix with proper dimensions
+            from sympy import S
+            if blocks[i*n + j] == S.Zero:
+                blocks[i*n + j] = ZeroMatrix(A.rowblocksizes[i], B.colblocksizes[j])
     return BlockMatrix(Matrix(m, n, blocks))
```

Actually, let me reconsider. Looking at the traceback more carefully, the issue is in the `_blockmul` method itself. The patch should be applied there. Let me provide a complete, clean patch:

--- a/sympy/matrices/expressions/blockmatrix.py
+++ b/sympy/matrices/expressions/blockmatrix.py
@@ -89,13 +89,20 @@ class BlockMatrix(MatrixExpr):
         return [self.blocks[0, i].cols for i in range(self.blockshape[1])]
 
     def _blockmul(self, other):
+        from sympy.matrices.expressions.matexpr import ZeroMatrix
+        from sympy import S
         if (isinstance(other, BlockMatrix) and
                 self.colblocksizes == other.rowblocksizes):
-            block_mul = lambda i, j: Add(*[self.blocks[i, k]*other.blocks[k, j]
-                        for k in range(self.blockshape[1])])
-
-            return BlockMatrix(Matrix(self.blockshape[0], other.blockshape[1], block_mul))
-        return self * other
+            def block_mul(i, j):
+                result = Add(*[self.blocks[i, k]*other.blocks[k, j]
+                              for k in range(self.blockshape[1])])
+                # Convert scalar Zero to ZeroMatrix with proper dimensions
+                if result == S.Zero:
+                    return ZeroMatrix(self.rowblocksizes[i], other.colblocksizes[j])
+                return result
+            return BlockMatrix(Matrix(self.blockshape[0], other.blockshape[1], block_mul))
+        return self * other
 
     def _blockadd(self, other):
         if (isinstance(other, BlockMatrix)