diff --git a/sao/src/model.js b/sao/src/model.js
index 46d52a9f5a..c982d50e76 100644
--- a/sao/src/model.js
+++ b/sao/src/model.js
@@ -1792,6 +1792,12 @@
             return value;
         },
         set_client: function(record, value, force_change) {
+            if (value === null) {
+                value = [];
+            }
+            if (typeof(value) == 'string') {
+                value = [value];
+            }
             if (value) {
                 value = value.slice().sort();
             }
diff --git a/tryton/tryton/gui/window/view_form/model/field.py b/tryton/tryton/gui/window/view_form/model/field.py
index 71bf228152..656b843e02 100644
--- a/tryton/tryton/gui/window/view_form/model/field.py
+++ b/tryton/tryton/gui/window/view_form/model/field.py
@@ -243,6 +243,10 @@ class MultiSelectionField(Field):
         return value
 
     def set_client(self, record, value, force_change=False):
+        if value is None:
+            value = []
+        if isinstance(value, str):
+            value = [value]
         if value:
             value = sorted(value)
         super().set_client(record, value, force_change=force_change)