diff --git a/pymisp/abstract.py b/pymisp/abstract.py
index ff1f708..368242d 100644
--- a/pymisp/abstract.py
+++ b/pymisp/abstract.py
@@ -237,7 +237,7 @@ class AbstractMISP(MutableMapping, MISPFileCache, metaclass=ABCMeta):
         to_return = _int_to_str(to_return)
         return to_return
 
-    def to_json(self, sort_keys: bool = False, indent: Optional[int] = None):
+    def to_json(self, sort_keys: bool = False, indent: Optional[int] = None) -> str:
         """Dump recursively any class of type MISPAbstract to a json string"""
         return dumps(self, default=pymisp_json_default, sort_keys=sort_keys, indent=indent)