diff --git a/CHANGELOG b/CHANGELOG
index a9f335b0a5..23ee051039 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,4 @@
+* Allow PYSON Expression as key for PYSON In with dict object
 * Support Binary field in CSV import/export
 
 Version 6.0.0 - 2021-05-03
diff --git a/tryton/pyson.py b/tryton/pyson.py
index 0b28b61ddf..e36656aa24 100644
--- a/tryton/pyson.py
+++ b/tryton/pyson.py
@@ -485,11 +485,17 @@ class In(PYSON):
             assert obj.types().issubset({dict, list}), \
                 'obj must be a dict or a list'
             if obj.types() == {dict}:
-                assert isinstance(key, str), 'key must be a string'
+                if isinstance(key, PYSON):
+                    assert key.types() == {str}, 'key must be a string'
+                else:
+                    assert isinstance(key, str), 'key must be a string'
         else:
             assert isinstance(obj, (dict, list))
             if isinstance(obj, dict):
-                assert isinstance(key, str), 'key must be a string'
+                if isinstance(key, PYSON):
+                    assert key.types() == {str}, 'key must be a string'
+                else:
+                    assert isinstance(key, str), 'key must be a string'
         self._key = key
         self._obj = obj