TypeError: cannot concatenate 'str' and 'NoneType' objects

[Malathi15]

After the training I tried to run Random_clip_valid.py I got this error

2019-04-04 10:31:57.009582: I tensorflow/core/platform/cpu_feature_guard.cc:141] Your CPU supports instructions that this TensorFlow binary was not compiled to use: AVX2 FMA
2019-04-04 10:31:57.097067: I tensorflow/stream_executor/cuda/cuda_gpu_executor.cc:964] successful NUMA node read from SysFS had negative value (-1), but there must be at least one NUMA node, so returning NUMA node zero
2019-04-04 10:31:57.097460: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1432] Found device 0 with properties: 
name: GeForce GTX 1080 Ti major: 6 minor: 1 memoryClockRate(GHz): 1.582
pciBusID: 0000:01:00.0
totalMemory: 10.91GiB freeMemory: 10.36GiB
2019-04-04 10:31:57.097489: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1511] Adding visible gpu devices: 0
2019-04-04 10:31:57.281719: I tensorflow/core/common_runtime/gpu/gpu_device.cc:982] Device interconnect StreamExecutor with strength 1 edge matrix:
2019-04-04 10:31:57.281761: I tensorflow/core/common_runtime/gpu/gpu_device.cc:988]      0 
2019-04-04 10:31:57.281766: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1001] 0:   N 
2019-04-04 10:31:57.281951: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1115] Created TensorFlow device (/job:localhost/replica:0/task:0/device:GPU:0 with 10053 MB memory) -> physical GPU (device: 0, name: GeForce GTX 1080 Ti, pci bus id: 0000:01:00.0, compute capability: 6.1)
('total_para:', 78409573)
Traceback (most recent call last):
  File "Random_clip_valid.py", line 197, in <module>
    modelpath="models")
  File "Random_clip_valid.py", line 163, in test
    saver.restore(sess, modelpath + tf.train.latest_checkpoint("c3d_ucf_model-4999"))
TypeError: cannot concatenate 'str' and 'NoneType' objects

can you help me to slove this issue