diff --git a/firedrake/assemble.py b/firedrake/assemble.py
index 60c934b6c7..88d00c6db8 100644
--- a/firedrake/assemble.py
+++ b/firedrake/assemble.py
@@ -311,7 +311,8 @@ def __init__(self,
                  zero_bc_nodes=False,
                  diagonal=False,
                  weight=1.0,
-                 allocation_integral_types=None):
+                 allocation_integral_types=None,
+                 needs_zeroing=False):
         super().__init__(form, bcs=bcs, form_compiler_parameters=form_compiler_parameters)
         self._mat_type = mat_type
         self._sub_mat_type = sub_mat_type
@@ -321,6 +322,7 @@ def __init__(self,
         self._diagonal = diagonal
         self._weight = weight
         self._allocation_integral_types = allocation_integral_types
+        assert not needs_zeroing
 
     def allocate(self):
         rank = len(self._form.arguments())
@@ -1127,7 +1129,8 @@ def _apply_bc(self, tensor, bc):
         pass
 
     def _check_tensor(self, tensor):
-        pass
+        if not isinstance(tensor, op2.Global):
+            raise TypeError(f"Expecting a op2.Global, got {tensor!r}.")
 
     @staticmethod
     def _as_pyop2_type(tensor, indices=None):
@@ -1143,7 +1146,7 @@ class OneFormAssembler(ParloopFormAssembler):
 
     Parameters
     ----------
-    form : ufl.Form or slate.TensorBasehe
+    form : ufl.Form or slate.TensorBase
         1-form.
 
     Notes
@@ -1189,8 +1192,8 @@ def _apply_bc(self, tensor, bc):
             self._apply_dirichlet_bc(tensor, bc)
         elif isinstance(bc, EquationBCSplit):
             bc.zero(tensor)
-            type(self)(bc.f, bcs=bc.bcs, form_compiler_parameters=self._form_compiler_params, needs_zeroing=False,
-                       zero_bc_nodes=self._zero_bc_nodes, diagonal=self._diagonal).assemble(tensor=tensor)
+            get_assembler(bc.f, bcs=bc.bcs, form_compiler_parameters=self._form_compiler_params, needs_zeroing=False,
+                          zero_bc_nodes=self._zero_bc_nodes, diagonal=self._diagonal).assemble(tensor=tensor)
         else:
             raise AssertionError
 
diff --git a/firedrake/matrix_free/operators.py b/firedrake/matrix_free/operators.py
index 3ee448730e..29abca838a 100644
--- a/firedrake/matrix_free/operators.py
+++ b/firedrake/matrix_free/operators.py
@@ -10,6 +10,8 @@
 from firedrake.bcs import DirichletBC, EquationBCSplit
 from firedrake.petsc import PETSc
 from firedrake.utils import cached_property
+from firedrake.function import Function
+from firedrake.cofunction import Cofunction
 
 
 __all__ = ("ImplicitMatrixContext", )
@@ -107,23 +109,22 @@ def __init__(self, a, row_bcs=[], col_bcs=[],
 
         # create functions from test and trial space to help
         # with 1-form assembly
-        test_space, trial_space = [
-            a.arguments()[i].function_space() for i in (0, 1)
-        ]
-        from firedrake import function, cofunction
+        test_space, trial_space = (
+            arg.function_space() for arg in a.arguments()
+        )
         # Need a cofunction since y receives the assembled result of Ax
-        self._ystar = cofunction.Cofunction(test_space.dual())
-        self._y = function.Function(test_space)
-        self._x = function.Function(trial_space)
-        self._xstar = cofunction.Cofunction(trial_space.dual())
+        self._ystar = Cofunction(test_space.dual())
+        self._y = Function(test_space)
+        self._x = Function(trial_space)
+        self._xstar = Cofunction(trial_space.dual())
 
         # These are temporary storage for holding the BC
         # values during matvec application.  _xbc is for
         # the action and ._ybc is for transpose.
         if len(self.bcs) > 0:
-            self._xbc = cofunction.Cofunction(trial_space.dual())
+            self._xbc = Cofunction(trial_space.dual())
         if len(self.col_bcs) > 0:
-            self._ybc = cofunction.Cofunction(test_space.dual())
+            self._ybc = Cofunction(test_space.dual())
 
         # Get size information from template vecs on test and trial spaces
         trial_vec = trial_space.dof_dset.layout_vec
@@ -135,6 +136,11 @@ def __init__(self, a, row_bcs=[], col_bcs=[],
 
         self.action = action(self.a, self._x)
         self.actionT = action(self.aT, self._y)
+        # TODO prevent action from returning empty Forms
+        if self.action.empty():
+            self.action = Cofunction(test_space.dual())
+        if self.actionT.empty():
+            self.action = Cofunction(trial_space.dual())
 
         # For assembling action(f, self._x)
         self.bcs_action = []
@@ -170,7 +176,6 @@ def __init__(self, a, row_bcs=[], col_bcs=[],
 
     @cached_property
     def _diagonal(self):
-        from firedrake import Cofunction
         assert self.on_diag
         return Cofunction(self._x.function_space().dual())