diff --git a/src/ifcopenshell-python/ifcopenshell/express/implementation.py b/src/ifcopenshell-python/ifcopenshell/express/implementation.py
index a500b41b2f..2e32e38829 100644
--- a/src/ifcopenshell-python/ifcopenshell/express/implementation.py
+++ b/src/ifcopenshell-python/ifcopenshell/express/implementation.py
@@ -159,6 +159,8 @@ class Implementation(codegen.Base):
                             "type": arg["full_type"].replace("::Value", ""),
                             "non_optional_type": arg["non_optional_type"].replace("::Value", ""),
                             "star_if_optional": "*" if "boost::optional" in arg["full_type"] else "",
+                            "check_optional_set_begin": "if (v) {" if "boost::optional" in arg["full_type"] else "",
+                            "check_optional_set_end": "}" if "boost::optional" in arg["full_type"] else "",
                         },
                     )
 
diff --git a/src/ifcopenshell-python/ifcopenshell/express/templates.py b/src/ifcopenshell-python/ifcopenshell/express/templates.py
index f52580f516..3b0b043e9f 100644
--- a/src/ifcopenshell-python/ifcopenshell/express/templates.py
+++ b/src/ifcopenshell-python/ifcopenshell/express/templates.py
@@ -259,13 +259,13 @@ get_attr_stmt_nested_array = "%(null_check)s aggregate_of_aggregate_of_instance:
 get_inverse = "return data_->getInverse(%(schema_name_upper)s_%(type)s_type, %(index)d)->as<%(type)s>();"
 
 set_attr_stmt = (
-    "{IfcWrite::IfcWriteArgument* attr = new IfcWrite::IfcWriteArgument();attr->set(%(star_if_optional)sv"
-    + ");data_->setArgument(%(index)d,attr);}"
+    "{IfcWrite::IfcWriteArgument* attr = new IfcWrite::IfcWriteArgument();%(check_optional_set_begin)sattr->set(%(star_if_optional)sv"
+    + ");%(check_optional_set_end)sdata_->setArgument(%(index)d,attr);}"
 )
-set_attr_stmt_enum = "{IfcWrite::IfcWriteArgument* attr = new IfcWrite::IfcWriteArgument();attr->set(IfcWrite::IfcWriteArgument::EnumerationReference(%(star_if_optional)sv,%(non_optional_type)s::ToString(%(star_if_optional)sv)));data_->setArgument(%(index)d,attr);}"
+set_attr_stmt_enum = "{IfcWrite::IfcWriteArgument* attr = new IfcWrite::IfcWriteArgument();%(check_optional_set_begin)sattr->set(IfcWrite::IfcWriteArgument::EnumerationReference(%(star_if_optional)sv,%(non_optional_type)s::ToString(%(star_if_optional)sv)));%(check_optional_set_end)sdata_->setArgument(%(index)d,attr);}"
 set_attr_stmt_array = (
-    "{IfcWrite::IfcWriteArgument* attr = new IfcWrite::IfcWriteArgument();attr->set((%(star_if_optional)sv)->generalize()"
-    + ");data_->setArgument(%(index)d,attr);}"
+    "{IfcWrite::IfcWriteArgument* attr = new IfcWrite::IfcWriteArgument();%(check_optional_set_begin)sattr->set((%(star_if_optional)sv)->generalize()"
+    + ");%(check_optional_set_end)sdata_->setArgument(%(index)d,attr);}"
 )
 
 constructor_stmt = (