diff --git a/lmformatenforcer/jsonschemaparser.py b/lmformatenforcer/jsonschemaparser.py
index 4fbb81f..9c19f7c 100644
--- a/lmformatenforcer/jsonschemaparser.py
+++ b/lmformatenforcer/jsonschemaparser.py
@@ -274,7 +274,7 @@ def get_allowed_characters(self) -> str:
         )
         required_keys = self.schema_object.required or []
         can_end = set(self.existing_keys).issuperset(required_keys)
-        can_parse_key = self.is_dictionary or set(possible_keys).issuperset(
+        can_parse_key = self.is_dictionary or set(possible_keys).difference(
             self.existing_keys
         )
 
diff --git a/tests/common.py b/tests/common.py
index c2d8dcd..a7d2d71 100644
--- a/tests/common.py
+++ b/tests/common.py
@@ -2,6 +2,10 @@
 from lmformatenforcer.exceptions import LMFormatEnforcerException
 
 
+class CharacterNotAllowedException(LMFormatEnforcerException):
+    pass
+
+
 def assert_parser_with_string(string: str, parser: CharacterLevelParser, expect_success: bool):
     for idx, character in enumerate(string):
         try:
@@ -9,7 +13,7 @@ def assert_parser_with_string(string: str, parser: CharacterLevelParser, expect_
                 parser = parser.add_character(character)
             else:
                 if expect_success:
-                    raise ValueError(f"Parser does not allow '{character}' at index {idx}")
+                    raise CharacterNotAllowedException(f"Parser does not allow '{character}' at index {idx}")
                 else:
                     return  # Success
         except LMFormatEnforcerException:
diff --git a/tests/test_jsonschemaparser.py b/tests/test_jsonschemaparser.py
index 86973f0..11d0f44 100644
--- a/tests/test_jsonschemaparser.py
+++ b/tests/test_jsonschemaparser.py
@@ -8,7 +8,7 @@
 
 from lmformatenforcer.exceptions import LMFormatEnforcerException
 
-from .common import assert_parser_with_string
+from .common import assert_parser_with_string, CharacterNotAllowedException
 
 
 def _test_json_schema_parsing_with_string(string: str, schema_dict: dict, expect_success: bool):
@@ -210,4 +210,15 @@ def test_string_escaping():
     # Unicode digit outside of hex range
     test_string = f'{{"num":1,"message":"hello {BACKSLASH}uf9fP world"}}'
     _test_json_schema_parsing_with_string(test_string, SampleModel.schema(), False)
+
+
+def test_comma_after_all_object_keys_fails():
+    class SomeSchema(BaseModel):
+        key: str
+
+    test_string = '{"key": "val",'
+    with pytest.raises(CharacterNotAllowedException):
+        _test_json_schema_parsing_with_string(test_string, SomeSchema.schema(), True)
+    
+    
     
\ No newline at end of file