diff --git a/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py b/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py
index 687dad096b..01a68e6a8e 100644
--- a/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py
+++ b/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py
@@ -388,11 +388,15 @@ def eval(self, expression: str) -> Any:
             from System.Globalization import CultureInfo
 
             original_culture = CultureInfo.CurrentCulture
-            CultureInfo.CurrentCulture = CultureInfo("en-US")
+            original_ui_culture = CultureInfo.CurrentUICulture
+            en_us_culture = CultureInfo("en-US")
+            CultureInfo.CurrentCulture = en_us_culture
+            CultureInfo.CurrentUICulture = en_us_culture
             try:
                 return engine.eval(formula, symbols=symbols)
             finally:
                 CultureInfo.CurrentCulture = original_culture
+                CultureInfo.CurrentUICulture = original_ui_culture
         except ValueError as e:
             error_msg = str(e)
             # Handle undefined variable errors gracefully by returning None
diff --git a/python/packages/declarative/tests/test_powerfx_yaml_compatibility.py b/python/packages/declarative/tests/test_powerfx_yaml_compatibility.py
index 9591dc05cb..8ea3c3af57 100644
--- a/python/packages/declarative/tests/test_powerfx_yaml_compatibility.py
+++ b/python/packages/declarative/tests/test_powerfx_yaml_compatibility.py
@@ -493,6 +493,31 @@ async def test_undefined_nested_variable_returns_none(self, mock_state):
         result = state.eval("=Local.Something.Nested.Deep")
         assert result is None
 
+    async def test_undefined_variable_returns_none_with_non_english_ui_culture(self, mock_state):
+        """Test that undefined variables return None even when CurrentUICulture is non-English.
+
+        Regression test for #4321: on non-English systems, CurrentUICulture causes
+        PowerFx to emit localized error messages that don't match the English
+        string guards ("isn't recognized", "Name isn't valid"), crashing the workflow.
+        The fix sets CurrentUICulture to en-US alongside CurrentCulture before eval.
+        """
+        from System.Globalization import CultureInfo
+
+        state = DeclarativeWorkflowState(mock_state)
+        state.initialize()
+
+        # Simulate a non-English UI culture (e.g. Italian)
+        original_ui_culture = CultureInfo.CurrentUICulture
+        CultureInfo.CurrentUICulture = CultureInfo("it-IT")
+        try:
+            # Should return None, not raise ValueError with Italian error text
+            result = state.eval("=Local.StatusConversationId")
+            assert result is None
+            # Verify the production code restored CurrentUICulture after eval
+            assert str(CultureInfo.CurrentUICulture) == str(CultureInfo("it-IT"))
+        finally:
+            CultureInfo.CurrentUICulture = original_ui_culture
+
 
 class TestStringInterpolation:
     """Test string interpolation patterns."""