diff --git a/nni/common/hpo_utils.py b/nni/common/hpo_utils.py
index f99e97eff1..3fb8cc1b6f 100644
--- a/nni/common/hpo_utils.py
+++ b/nni/common/hpo_utils.py
@@ -47,6 +47,11 @@ def validate_search_space(
             raise ValueError(f'search space "{name}"\'s value is not a list : {spec}')
 
         if type_ == 'choice':
+            if not all(isinstance(arg, (float, int, str)) for arg in args):
+                # FIXME: need further check for each algorithm which types are actually supported
+                # for now validation only prints warning so it doesn't harm
+                if not isinstance(args[0], dict) or '_name' not in args[0]:  # not nested search space
+                    raise ValueError(f'search space "{name}" (choice) should only contain numbers or strings : {spec}')
             continue
 
         if type_.startswith('q'):
diff --git a/test/ut/sdk/test_hpo_utils.py b/test/ut/sdk/test_hpo_utils.py
index e0f6d12294..c1204d0b9a 100644
--- a/test/ut/sdk/test_hpo_utils.py
+++ b/test/ut/sdk/test_hpo_utils.py
@@ -16,12 +16,22 @@
     'choice': good['choice'],
     'randint': good['randint'],
 }
+good_nested = {
+    'outer': {
+        '_type': 'choice',
+        '_value': [
+            { '_name': 'empty' },
+            { '_name': 'a', 'a_1': { '_type': 'choice', '_value': ['a', 'b'] } }
+        ]
+    }
+}
 
 bad_type = 'x'
 bad_spec_type = { 'x': [1, 2, 3] }
 bad_fields = { 'x': { 'type': 'choice', 'value': ['a', 'b'] } }
 bad_type_name = { 'x': { '_type': 'choic', '_value': ['a'] } }
 bad_value = { 'x': { '_type': 'choice', '_value': 'ab' } }
+bad_choice_args = { 'x': { '_type': 'choice', 'value': [ 'a', object() ] } }
 bad_2_args = { 'x': { '_type': 'randint', '_value': [1, 2, 3] } }
 bad_3_args = { 'x': { '_type': 'quniform', '_value': [0] } }
 bad_int_args = { 'x': { '_type': 'randint', '_value': [1.0, 2.0] } }
@@ -32,11 +42,13 @@
 
 def test_hpo_utils():
     assert validate_search_space(good, raise_exception=False)
+    assert validate_search_space(good_nested, raise_exception=False)
     assert not validate_search_space(bad_type, raise_exception=False)
     assert not validate_search_space(bad_spec_type, raise_exception=False)
     assert not validate_search_space(bad_fields, raise_exception=False)
     assert not validate_search_space(bad_type_name, raise_exception=False)
     assert not validate_search_space(bad_value, raise_exception=False)
+    assert not validate_search_space(bad_choice_args, raise_exception=False)
     assert not validate_search_space(bad_2_args, raise_exception=False)
     assert not validate_search_space(bad_3_args, raise_exception=False)
     assert not validate_search_space(bad_int_args, raise_exception=False)