diff --git a/src/dbus_fast/_private/unmarshaller.pxd b/src/dbus_fast/_private/unmarshaller.pxd
index c95f85e..d6da01c 100644
--- a/src/dbus_fast/_private/unmarshaller.pxd
+++ b/src/dbus_fast/_private/unmarshaller.pxd
@@ -5,6 +5,16 @@ import cython
 from ..signature import SignatureType
 
 
+cdef str HEADER_DESTINATION
+cdef str HEADER_PATH
+cdef str HEADER_INTERFACE
+cdef str HEADER_MEMBER
+cdef str HEADER_ERROR_NAME
+cdef str HEADER_REPLY_SERIAL
+cdef str HEADER_SENDER
+cdef str HEADER_SIGNATURE
+
+
 cdef unsigned int UINT32_SIZE
 cdef unsigned int HEADER_ARRAY_OF_STRUCT_SIGNATURE_POSITION
 cdef unsigned int HEADER_SIGNATURE_SIZE
diff --git a/src/dbus_fast/_private/unmarshaller.py b/src/dbus_fast/_private/unmarshaller.py
index d7737ef..b6493e5 100644
--- a/src/dbus_fast/_private/unmarshaller.py
+++ b/src/dbus_fast/_private/unmarshaller.py
@@ -57,6 +57,7 @@ HEADER_MEMBER = HeaderField.MEMBER.name
 HEADER_ERROR_NAME = HeaderField.ERROR_NAME.name
 HEADER_REPLY_SERIAL = HeaderField.REPLY_SERIAL.name
 HEADER_SENDER = HeaderField.SENDER.name
+HEADER_SIGNATURE = HeaderField.SIGNATURE.name
 
 READER_TYPE = Callable[["Unmarshaller", SignatureType], Any]
 
@@ -391,7 +392,7 @@ class Unmarshaller:
         self._pos = HEADER_ARRAY_OF_STRUCT_SIGNATURE_POSITION
         header_fields = self.header_fields(self._header_len)
         self._pos += -self._pos & 7  # align 8
-        tree = get_signature_tree(header_fields.get(HeaderField.SIGNATURE.name, ""))
+        tree = get_signature_tree(header_fields.get(HEADER_SIGNATURE, ""))
         self._message = Message(
             destination=header_fields.get(HEADER_DESTINATION),
             path=header_fields.get(HEADER_PATH),