diff --git a/src/dbus_fast/_private/marshaller.pxd b/src/dbus_fast/_private/marshaller.pxd
index 6995d687..958b8920 100644
--- a/src/dbus_fast/_private/marshaller.pxd
+++ b/src/dbus_fast/_private/marshaller.pxd
@@ -27,7 +27,7 @@ cdef class Marshaller:
         signature_len=cython.uint,
         written=cython.uint,
     )
-    cpdef write_string(self, object value, object _type)
+    cpdef write_string(self, str value, object _type)
 
     @cython.locals(
         signature_len=cython.uint,
@@ -59,6 +59,8 @@ cdef class Marshaller:
 
     @cython.locals(
         written=cython.uint,
+        signature=cython.str,
+        signature_bytes=cython.bytes,
     )
     cpdef write_variant(self, object variant, object type)
 
diff --git a/src/dbus_fast/_private/marshaller.py b/src/dbus_fast/_private/marshaller.py
index ecfa2597..8ec71beb 100644
--- a/src/dbus_fast/_private/marshaller.py
+++ b/src/dbus_fast/_private/marshaller.py
@@ -63,7 +63,9 @@ def write_string(self, value, type_: SignatureType) -> int:
         return written
 
     def write_variant(self, variant: Variant, type_: SignatureType) -> int:
-        written = self._write_signature(variant.signature.encode())
+        signature = variant.signature
+        signature_bytes = signature.encode()
+        written = self._write_signature(signature_bytes)
         written += self._write_single(variant.type, variant.value)
         return written