diff --git a/src/gotenberg_client/options.py b/src/gotenberg_client/options.py
index e03d761..3ee06e1 100644
--- a/src/gotenberg_client/options.py
+++ b/src/gotenberg_client/options.py
@@ -175,7 +175,7 @@ def to_form(self, name: str) -> Dict[str, str]:
         if self.unit == MarginUnitType.Undefined:
             return optional_to_form(self.value, name)
         else:
-            return optional_to_form(f"{self.value}{self.unit}", name)
+            return optional_to_form(f"{self.value}{self.unit.value}", name)
 
 
 @dataclasses.dataclass