Introduction When using OpenCV’s Deep Neural Network (DNN) module with Compute Unified Device Architecture CUDA, you may encounter the following error: (-215:Assertion failed) preferableBackend != DNN_BACKEND_CUDA || IS_DNN_CUDA_TARGET(preferableTarget) This error occurs when OpenCV’s DNN module fails to properly configure CUDA as the preferred backend for inference. In this article, we will explore why this happens, how we fixed it, and why we had to rebuild OpenCV. Understanding the Error This assertion failure happens in OpenCV when: CUDA is selected as the backend (DNN_BACKEND_CUDA), but The target (DNN_TARGET_CUDA) is not properly recognized as a CUDA-compatible target. The primary causes of this issue include: Missing cuDNN: OpenCV’s DNN module requires cuDNN to run inference on GPUs. Incorrect OpenCV Build: OpenCV must be built with CUDA and cuDNN support. Incompatible CUDA or cuDNN Versions: