diff --git a/tests/engine/test_arg_utils.py b/tests/engine/test_arg_utils.py
index 3208d6bb48bdc..8dd200b35d0f3 100644
--- a/tests/engine/test_arg_utils.py
+++ b/tests/engine/test_arg_utils.py
@@ -1,6 +1,8 @@
+from argparse import ArgumentTypeError
+
 import pytest
 
-from vllm.engine.arg_utils import EngineArgs
+from vllm.engine.arg_utils import EngineArgs, nullable_kvs
 from vllm.utils import FlexibleArgumentParser
 
 
@@ -13,6 +15,10 @@
         "image": 16,
         "video": 2
     }),
+    ("Image=16, Video=2", {
+        "image": 16,
+        "video": 2
+    }),
 ])
 def test_limit_mm_per_prompt_parser(arg, expected):
     parser = EngineArgs.add_cli_args(FlexibleArgumentParser())
@@ -22,3 +28,15 @@ def test_limit_mm_per_prompt_parser(arg, expected):
         args = parser.parse_args(["--limit-mm-per-prompt", arg])
 
     assert args.limit_mm_per_prompt == expected
+
+
+@pytest.mark.parametrize(
+    ("arg"),
+    [
+        "image",  # Missing =
+        "image=4,image=5",  # Conflicting values
+        "image=video=4"  # Too many = in tokenized arg
+    ])
+def test_bad_nullable_kvs(arg):
+    with pytest.raises(ArgumentTypeError):
+        nullable_kvs(arg)
diff --git a/vllm/engine/arg_utils.py b/vllm/engine/arg_utils.py
index b5eba9ca3727a..35013eedea9c6 100644
--- a/vllm/engine/arg_utils.py
+++ b/vllm/engine/arg_utils.py
@@ -44,22 +44,36 @@ def nullable_str(val: str):
 
 
 def nullable_kvs(val: str) -> Optional[Mapping[str, int]]:
+    """Parses a string containing comma separate key [str] to value [int]
+    pairs into a dictionary.
+
+    Args:
+        val: String value to be parsed.
+
+    Returns:
+        Dictionary with parsed values.
+    """
     if len(val) == 0:
         return None
 
     out_dict: Dict[str, int] = {}
     for item in val.split(","):
-        try:
-            key, value = item.split("=")
-        except TypeError as exc:
-            msg = "Each item should be in the form KEY=VALUE"
-            raise ValueError(msg) from exc
+        kv_parts = [part.lower().strip() for part in item.split("=")]
+        if len(kv_parts) != 2:
+            raise argparse.ArgumentTypeError(
+                "Each item should be in the form KEY=VALUE")
+        key, value = kv_parts
 
         try:
-            out_dict[key] = int(value)
+            parsed_value = int(value)
         except ValueError as exc:
             msg = f"Failed to parse value of item {key}={value}"
-            raise ValueError(msg) from exc
+            raise argparse.ArgumentTypeError(msg) from exc
+
+        if key in out_dict and out_dict[key] != parsed_value:
+            raise argparse.ArgumentTypeError(
+                f"Conflicting values specified for key: {key}")
+        out_dict[key] = parsed_value
 
     return out_dict