diff --git a/firedrake/external_operators/abstract_external_operators.py b/firedrake/external_operators/abstract_external_operators.py
index 3afc09f318..d653b25f4d 100644
--- a/firedrake/external_operators/abstract_external_operators.py
+++ b/firedrake/external_operators/abstract_external_operators.py
@@ -194,6 +194,8 @@ def assemble(self, assembly_opts=None):
             # Will also catch the case where wrong fct space
             if not isinstance(result, (Function, Cofunction)):
                 raise ValueError('External operators with one argument must result in a firedrake.Function or firedrake.Cofunction object!')
+            if not result.function_space() == self.arguments()[0].function_space().dual():
+                raise ValueError('External operator assembly result is in the wrong function space.')
         elif len(self.arguments()) == 2:
             if not isinstance(result, MatrixBase):
                 raise ValueError('External operators with two arguments must result in a firedrake.MatrixBase object!')