diff --git a/cpp/dolfinx/fem/assembler.h b/cpp/dolfinx/fem/assembler.h
index c53aef84b4..f1c6f6271a 100644
--- a/cpp/dolfinx/fem/assembler.h
+++ b/cpp/dolfinx/fem/assembler.h
@@ -15,6 +15,7 @@
 #include <cstdint>
 #include <dolfinx/common/types.h>
 #include <memory>
+#include <optional>
 #include <span>
 #include <vector>
 
@@ -37,10 +38,10 @@ make_coefficients_span(const std::map<std::pair<IntegralType, int>,
 {
   using Key = typename std::remove_reference_t<decltype(coeffs)>::key_type;
   std::map<Key, std::pair<std::span<const T>, int>> c;
-  std::ranges::transform(coeffs, std::inserter(c, c.end()),
-                         [](auto& e) -> typename decltype(c)::value_type {
-                           return {e.first, {e.second.first, e.second.second}};
-                         });
+  std::ranges::transform(
+      coeffs, std::inserter(c, c.end()),
+      [](auto& e) -> typename decltype(c)::value_type
+      { return {e.first, {e.second.first, e.second.second}}; });
   return c;
 }
 
@@ -418,30 +419,26 @@ void set_diagonal(
 template <dolfinx::scalar T, std::floating_point U>
 void set_bc(std::span<T> b,
             const std::vector<std::shared_ptr<const DirichletBC<T, U>>>& bcs,
-            std::span<const T> x0, T scale = 1)
+            std::optional<std::span<const T>> x0, T scale = 1)
 {
-  if (b.size() > x0.size())
-    throw std::runtime_error("Size mismatch between b and x0 vectors.");
-  for (auto& bc : bcs)
+  if (x0.has_value())
   {
-    assert(bc);
-    bc->set(b, x0, scale);
-  }
-}
 
-/// Set bc values in owned (local) part of the vector, multiplied by
-/// 'scale'. The bcs should be on (sub-)spaces of the form L that b
-/// represents.
-template <dolfinx::scalar T, std::floating_point U>
-void set_bc(std::span<T> b,
-            const std::vector<std::shared_ptr<const DirichletBC<T, U>>>& bcs,
-            T scale = 1)
-{
-  for (auto& bc : bcs)
+    if (b.size() > x0.value().size())
+      throw std::runtime_error("Size mismatch between b and x0 vectors.");
+    for (auto& bc : bcs)
+    {
+      assert(bc);
+      bc->set(b, x0.value(), scale);
+    }
+  }
+  else
   {
-    assert(bc);
-    bc->set(b, scale);
+    for (auto& bc : bcs)
+    {
+      assert(bc);
+      bc->set(b, scale);
+    }
   }
 }
-
 } // namespace dolfinx::fem
diff --git a/python/dolfinx/fem/assemble.py b/python/dolfinx/fem/assemble.py
index d3bb918e80..3ba3915c74 100644
--- a/python/dolfinx/fem/assemble.py
+++ b/python/dolfinx/fem/assemble.py
@@ -359,8 +359,4 @@ def set_bc(
     calling this function is not required unless ghost entries need to
     be updated to the boundary condition value.
     """
-    _bcs = [bc._cpp_object for bc in bcs]
-    if x0 is None:
-        _cpp.fem.set_bc(b, _bcs, scale)
-    else:
-        _cpp.fem.set_bc(b, _bcs, x0, scale)
+    _cpp.fem.set_bc(b, [bc._cpp_object for bc in bcs], x0, scale)
diff --git a/python/dolfinx/wrappers/assemble.cpp b/python/dolfinx/wrappers/assemble.cpp
index d216039d08..4f71eb5c67 100644
--- a/python/dolfinx/wrappers/assemble.cpp
+++ b/python/dolfinx/wrappers/assemble.cpp
@@ -193,7 +193,8 @@ void declare_assembly_functions(nb::module_& m)
       nb::arg("form"), "Pack coefficients for a Form.");
   m.def(
       "pack_constants",
-      [](const dolfinx::fem::Form<T, U>& form) {
+      [](const dolfinx::fem::Form<T, U>& form)
+      {
         return dolfinx_wrappers::as_nbarray(dolfinx::fem::pack_constants(form));
       },
       nb::arg("form"), "Pack constants for a Form.");
@@ -417,21 +418,23 @@ void declare_assembly_functions(nb::module_& m)
       [](nb::ndarray<T, nb::ndim<1>, nb::c_contig> b,
          const std::vector<
              std::shared_ptr<const dolfinx::fem::DirichletBC<T, U>>>& bcs,
-         nb::ndarray<const T, nb::ndim<1>, nb::c_contig> x0, T scale)
+         std::optional<nb::ndarray<const T, nb::ndim<1>, nb::c_contig>> x0,
+         T scale)
       {
-        dolfinx::fem::set_bc<T>(std::span(b.data(), b.size()), bcs,
-                                std::span(x0.data(), x0.shape(0)), scale);
+        if (x0.has_value())
+        {
+          dolfinx::fem::set_bc<T>(
+              std::span(b.data(), b.size()), bcs,
+              std::span(x0.value().data(), x0.value().shape(0)), scale);
+        }
+        else
+        {
+          dolfinx::fem::set_bc<T>(std::span(b.data(), b.size()), bcs,
+                                  std::nullopt, scale);
+        }
       },
       nb::arg("b").noconvert(), nb::arg("bcs"), nb::arg("x0").noconvert(),
       nb::arg("scale"));
-  m.def(
-      "set_bc",
-      [](nb::ndarray<T, nb::ndim<1>, nb::c_contig> b,
-         const std::vector<
-             std::shared_ptr<const dolfinx::fem::DirichletBC<T, U>>>& bcs,
-         T scale)
-      { dolfinx::fem::set_bc<T>(std::span(b.data(), b.size()), bcs, scale); },
-      nb::arg("b").noconvert(), nb::arg("bcs"), nb::arg("scale"));
 }
 
 } // namespace