diff --git a/litellm/llms/black_forest_labs/image_generation/transformation.py b/litellm/llms/black_forest_labs/image_generation/transformation.py
index 88ad9a57c18..76267491e3e 100644
--- a/litellm/llms/black_forest_labs/image_generation/transformation.py
+++ b/litellm/llms/black_forest_labs/image_generation/transformation.py
@@ -127,7 +127,9 @@ class BlackForestLabsImageGenerationConfig(BaseImageGenerationConfig):
                 optional_params["width"] = width
                 optional_params["height"] = height
             except ValueError:
-                pass  # Ignore invalid size format
+                raise ValueError(
+                    f"Invalid size format: '{size}'. Expected format 'WIDTHxHEIGHT' (e.g., '1024x1024')."
+                )
 
     def validate_environment(
         self,