diff --git a/proto/ei-scanner b/proto/ei-scanner
index 4aefd0a..11edb85 100755
--- a/proto/ei-scanner
+++ b/proto/ei-scanner
@@ -92,6 +92,10 @@ class Argument:
     For an argument referenced by another argument of type "new_id", this field
     points to the other argument that references this argument.
     """
+    allow_null: bool = attr.ib(default=False)
+    """
+    For an argument of type string, specify if the argument may be NULL.
+    """
 
     @property
     def signature(self) -> str:
@@ -137,6 +141,7 @@ class Argument:
         summary: str = "",
         enum: Optional["Enum"] = None,
         interface: Optional["Interface"] = None,
+        allow_null: bool = False,
     ) -> "Argument":
         return cls(
             name=name,
@@ -144,6 +149,7 @@ class Argument:
             summary=summary,
             enum=enum,
             interface=interface,
+            allow_null=allow_null,
         )
 
 
@@ -709,12 +715,15 @@ class ProtocolParser(xml.sax.handler.ContentHandler):
                         f"Failed to find enum '{intf.name}.{enum_name}'",
                         self.location,
                     )
+
+            allow_null = attrs.get("allow-null", "false") == "true"
             arg = Argument.create(
                 name=name,
                 protocol_type=proto_type,
                 summary=summary,
                 enum=enum,
                 interface=interface,
+                allow_null=allow_null,
             )
             self.current_message.add_argument(arg)
             if proto_type == "new_id":