unspecified driver error in cudaMalloc

hi , I have this wierd behaviour while allocatin Device memory using cudaMalloc

    cudaError_t error;	

10 float* gpuA;float* gpuB; float* gpuC;
error = cudaMalloc((void**)&gpuA, 512 * sizeof(float));
printf(“error :%s\n”,cudaGetErrorString(error));

error = cudaMalloc((void**)&gpuB, 512 * sizeof(float));
printf("error :%s\n",cudaGetErrorString(error));

the problem is that when line 10 is executed, “gpuA” gets an invalid value like 0x01 or 0x02 , which indicates that no allocation occurred , and the last error is “unspecified driver error”

can anyone help ?