diff --git a/guppylang/definition/custom.py b/guppylang/definition/custom.py
index 67789fe0..b2e86370 100644
--- a/guppylang/definition/custom.py
+++ b/guppylang/definition/custom.py
@@ -12,7 +12,7 @@
 from guppylang.definition.common import ParsableDef
 from guppylang.definition.value import CompiledCallableDef
 from guppylang.error import GuppyError, InternalGuppyError
-from guppylang.hugr_builder.hugr import Hugr, Node, OutPortV
+from guppylang.hugr_builder.hugr import Hugr, OutPortV
 from guppylang.nodes import GlobalCall
 from guppylang.tys.subst import Inst, Subst
 from guppylang.tys.ty import FunctionType, NoneType, Type, type_to_row
@@ -146,26 +146,22 @@ def load_with_args(
             )
         assert len(self.ty.params) == len(type_args)
 
-        # Find the module node by walking up the hierarchy
-        module: Node = dfg.node
-        while not isinstance(module.op, ops.Module):
-            if module.parent is None:
-                raise InternalGuppyError(
-                    "Encountered node that is not contained in a module."
-                )
-            module = module.parent
-
         # We create a `FunctionDef` that takes some inputs, compiles a call to the
-        # function, and returns the results.
-        def_node = graph.add_def(self.ty, module, self.name)
-        _, inp_ports = graph.add_input_with_ports(list(self.ty.inputs), def_node)
-        returns = self.compile_call(
-            inp_ports, type_args, DFContainer(def_node, {}), graph, globals, node
-        )
-        graph.add_output(returns, parent=def_node)
+        # function, and returns the results. If the function signature is polymorphic,
+        # we explicitly monomorphise here and invoke the call compiler with the
+        # inferred type args.
+        fun_ty = self.ty.instantiate(type_args)
+        def_node = graph.add_def(fun_ty, dfg.node, self.name)
+        with graph.parent(def_node):
+            _, inp_ports = graph.add_input_with_ports(list(fun_ty.inputs))
+            returns = self.compile_call(
+                inp_ports, type_args, DFContainer(def_node, {}), graph, globals, node
+            )
+            graph.add_output(returns)
 
-        # Finally, load the function into the local DFG
-        return graph.add_load_constant(def_node.out_port(0), dfg.node).out_port(0)
+        # Finally, load the function into the local DFG. We already monomorphised, so we
+        # can load with empty type args
+        return graph.add_load_function(def_node.out_port(0), [], dfg.node).out_port(0)
 
 
 class CustomCallChecker(ABC):
diff --git a/tests/integration/test_poly.py b/tests/integration/test_poly.py
index 471f4851..5edb4d79 100644
--- a/tests/integration/test_poly.py
+++ b/tests/integration/test_poly.py
@@ -3,6 +3,8 @@
 import pytest
 
 from guppylang.decorator import guppy
+from guppylang.definition.custom import CustomCallCompiler
+from guppylang.hugr_builder.hugr import OutPortV
 from guppylang.module import GuppyModule
 from guppylang.prelude.builtins import array
 from guppylang.prelude.quantum import qubit
@@ -274,6 +276,23 @@ def main() -> None:
     validate(module.compile())
 
 
+def test_custom_higher_order():
+    class CustomCompiler(CustomCallCompiler):
+        def compile(self, args: list[OutPortV]) -> list[OutPortV]:
+            return args
+
+    module = GuppyModule("test")
+    T = guppy.type_var(module, "T")
+
+    @guppy.custom(module, CustomCompiler())
+    def foo(x: T) -> T: ...
+
+    @guppy(module)
+    def main(x: int) -> int:
+        f: Callable[[int], int] = foo
+        return f(x)
+
+
 @pytest.mark.skip("Not yet supported")
 def test_higher_order_value(validate):
     module = GuppyModule("test")