diff --git a/asyncua/common/structures104.py b/asyncua/common/structures104.py
index ce98ac726..dd920ace3 100644
--- a/asyncua/common/structures104.py
+++ b/asyncua/common/structures104.py
@@ -526,11 +526,23 @@ class {name}({enum_type}):
     '''
 
 """
-
-    for sfield in edef.Fields:
-        name = clean_name(sfield.Name)
-        value = sfield.Value if not option_set else (1 << sfield.Value)
-        code += f"    {name} = {value}\n"
+    for n, sfield in enumerate(edef.Fields):
+        fieldname = clean_name(sfield.Name)
+        if hasattr(sfield, "Value"):
+            value = sfield.Value if not option_set else (1 << sfield.Value)
+        else:
+            # Some servers represent the datatype as StructureDefinition instead of EnumDefinition.
+            # In this case the Value attribute is missing and we must guess.
+            # XXX: Assuming that counting starts with 1 for enumerations, which is by no means guaranteed.
+            value = n+1 if not option_set else (1 << n)
+            if n==0:
+                _logger.warning(
+                    "%s type %s: guessing field values since the server does not provide them.",
+                    "OptionSet" if option_set else "Enumeration",
+                    name
+                )
+
+        code += f"    {fieldname} = {value}\n"
     return code
 
 
diff --git a/tests/test_common.py b/tests/test_common.py
index 6eb4a3722..e41ac0011 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -1280,6 +1280,27 @@ async def test_custom_option_set(opc):
     val = await var.read_value()
     assert val == (1 << 2) | (1 << 1)
 
+async def test_custom_option_set_as_structure_definition(opc):
+    """Option set defined via structure definition is understood.
+
+    This is arguably outside the standard but observed on some servers.
+    """
+    idx = 4
+    server = opc.opc
+    await new_enum(opc.opc, idx, "MyOptionSet", ["tata", "titi", "toto", "None"], True)
+    sdef = ua.StructureDefinition()
+    for name in ["tata", "titi", "toto", "None"]:
+        field = ua.StructureField()
+        field.Name = name
+        sdef.Fields.append(field)
+    dtype = await server.nodes.option_set_type.add_data_type(idx, name)
+    await dtype.write_data_type_definition(sdef)
+
+    await opc.opc.load_data_type_definitions()
+    assert ua.MyOptionSet.toto | ua.MyOptionSet.titi == ua.MyOptionSet((1 << 2) | (1 << 1))
+    var = await opc.opc.nodes.objects.add_variable(idx, "my_option", ua.MyOptionSet.toto | ua.MyOptionSet.titi)
+    val = await var.read_value()
+    assert val == (1 << 2) | (1 << 1)
 
 async def test_custom_struct_(opc):
     idx = 4