diff --git a/pybamm/expression_tree/symbol.py b/pybamm/expression_tree/symbol.py
index 1bea14eeb1..0bb30419e6 100644
--- a/pybamm/expression_tree/symbol.py
+++ b/pybamm/expression_tree/symbol.py
@@ -112,9 +112,16 @@ def is_matrix_x(expr, x):
 
     if is_constant(expr):
         result = expr.evaluate_ignoring_errors(t=None)
-        return (issparse(result) and np.all(result.__dict__["data"] == x)) or (
-            isinstance(result, np.ndarray) and np.all(result == x)
-        )
+        return (
+            issparse(result)
+            and (
+                (x == 0 and np.prod(len(result.__dict__["data"])) == 0)
+                or (
+                    len(result.__dict__["data"]) == np.prod(result.shape)
+                    and np.all(result.__dict__["data"] == x)
+                )
+            )
+        ) or (isinstance(result, np.ndarray) and np.all(result == x))
     else:
         return False
 
diff --git a/pybamm/spatial_methods/finite_volume.py b/pybamm/spatial_methods/finite_volume.py
index d9851d1d17..65e6fad18d 100644
--- a/pybamm/spatial_methods/finite_volume.py
+++ b/pybamm/spatial_methods/finite_volume.py
@@ -230,7 +230,7 @@ def laplacian(self, symbol, discretised_symbol, boundary_conditions):
         return self.divergence(grad, grad, boundary_conditions)
 
     def integral(self, child, discretised_child, integration_dimension):
-        """Vector-vector dot product to implement the integral operator. """
+        """Vector-vector dot product to implement the integral operator."""
         integration_vector = self.definite_integral_matrix(
             child, integration_dimension=integration_dimension
         )
@@ -315,9 +315,9 @@ def definite_integral_matrix(
             # repeat matrix for each node in higher dimensions
             third_dim_repeats = self._get_auxiliary_domain_repeats(
                 {
-                    k: v for k, v in domains.items() if (
-                        k == "tertiary" or k == "quaternary"
-                    )
+                    k: v
+                    for k, v in domains.items()
+                    if (k == "tertiary" or k == "quaternary")
                 }
             )
             # generate full matrix from the submatrix
@@ -330,7 +330,7 @@ def definite_integral_matrix(
         return pybamm.Matrix(csr_matrix(matrix))
 
     def indefinite_integral(self, child, discretised_child, direction):
-        """Implementation of the indefinite integral operator. """
+        """Implementation of the indefinite integral operator."""
 
         # Different integral matrix depending on whether the integrand evaluates on
         # edges or nodes