diff --git a/examples/offline_inference/image_to_image/image_edit.py b/examples/offline_inference/image_to_image/image_edit.py
index 96ea35a4b..2f1a8166d 100644
--- a/examples/offline_inference/image_to_image/image_edit.py
+++ b/examples/offline_inference/image_to_image/image_edit.py
@@ -110,7 +110,7 @@ def parse_args() -> argparse.Namespace:
     parser.add_argument(
         "--negative_prompt",
         type=str,
-        default=" ",
+        default=None,
         required=False,
     )
     parser.add_argument(
diff --git a/examples/offline_inference/text_to_image/text_to_image.py b/examples/offline_inference/text_to_image/text_to_image.py
index fdb7c8a0a..b07dbb27a 100644
--- a/examples/offline_inference/text_to_image/text_to_image.py
+++ b/examples/offline_inference/text_to_image/text_to_image.py
@@ -24,7 +24,9 @@ def parse_args() -> argparse.Namespace:
     )
     parser.add_argument("--prompt", default="a cup of coffee on the table", help="Text prompt for image generation.")
     parser.add_argument(
-        "--negative_prompt", default="", help="negative prompt for classifier-free conditional guidance."
+        "--negative_prompt",
+        default=None,
+        help="negative prompt for classifier-free conditional guidance.",
     )
     parser.add_argument("--seed", type=int, default=142, help="Random seed for deterministic results.")
     parser.add_argument(
diff --git a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image.py b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image.py
index 87c7c2f73..5ae8228a0 100644
--- a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image.py
+++ b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image.py
@@ -642,8 +642,8 @@ class QwenImagePipeline(
     def forward(
         self,
         req: OmniDiffusionRequest,
-        prompt: str | list[str] = "",
-        negative_prompt: str | list[str] = "",
+        prompt: str | list[str] | None = None,
+        negative_prompt: str | list[str] | None = None,
         true_cfg_scale: float = 4.0,
         height: int | None = None,
         width: int | None = None,
@@ -665,8 +665,8 @@ class QwenImagePipeline(
         # # TODO: only support single prompt now
         # if req.prompt is not None:
         #     prompt = req.prompt[0] if isinstance(req.prompt, list) else req.prompt
-        prompt = req.prompt if req.prompt is not None else prompt
-        negative_prompt = req.negative_prompt if req.negative_prompt is not None else negative_prompt
+        prompt = req.prompt
+        negative_prompt = req.negative_prompt
         height = req.height or self.default_sample_size * self.vae_scale_factor
         width = req.width or self.default_sample_size * self.vae_scale_factor
         num_inference_steps = req.num_inference_steps or num_inference_steps
diff --git a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit.py b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit.py
index 901625e34..4927ca3ac 100644
--- a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit.py
+++ b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit.py
@@ -695,8 +695,8 @@ class QwenImageEditPipeline(
     def forward(
         self,
         req: OmniDiffusionRequest,
-        prompt: str | list[str] = "",
-        negative_prompt: str | list[str] = "",
+        prompt: str | list[str] | None = None,
+        negative_prompt: str | list[str] | None = None,
         image: PIL.Image.Image | torch.Tensor | None = None,
         true_cfg_scale: float = 4.0,
         height: int | None = None,
@@ -717,8 +717,8 @@ class QwenImageEditPipeline(
         max_sequence_length: int = 512,
     ) -> DiffusionOutput:
         """Forward pass for image editing."""
-        prompt = req.prompt if req.prompt is not None else prompt
-        negative_prompt = req.negative_prompt if req.negative_prompt is not None else negative_prompt
+        prompt = req.prompt
+        negative_prompt = req.negative_prompt
 
         # Get preprocessed image from request (pre-processing is done in DiffusionEngine)
         if hasattr(req, "preprocessed_image"):
diff --git a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py
index 6df502d48..9eeaed5f9 100644
--- a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py
+++ b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py
@@ -633,8 +633,8 @@ class QwenImageEditPlusPipeline(nn.Module, SupportImageInput):
     def forward(
         self,
         req: OmniDiffusionRequest,
-        prompt: str | list[str] = "",
-        negative_prompt: str | list[str] = "",
+        prompt: str | list[str] | None = None,
+        negative_prompt: str | list[str] | None = None,
         image: PIL.Image.Image | list[PIL.Image.Image] | torch.Tensor | None = None,
         true_cfg_scale: float = 4.0,
         height: int | None = None,
@@ -655,8 +655,8 @@ class QwenImageEditPlusPipeline(nn.Module, SupportImageInput):
         max_sequence_length: int = 512,
     ) -> DiffusionOutput:
         """Forward pass for image editing with support for multiple images."""
-        prompt = req.prompt if req.prompt is not None else prompt
-        negative_prompt = req.negative_prompt if req.negative_prompt is not None else negative_prompt
+        prompt = req.prompt
+        negative_prompt = req.negative_prompt
 
         # Get preprocessed images from request (pre-processing is done in DiffusionEngine)
         if hasattr(req, "vae_images") and hasattr(req, "condition_images"):