SVR auto_train error Assertion failed (sv_count != 0) in do_train

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I am using CVSVM for regression and it worked fine when I tune the parameters manually. But when I use the grid search method to optimize the parameters: 1.it kept giving me the "Assertion failed (sv_count != 0) in do_train" error when I used it on a large dataset (about 10000 instances and 10 features). 2. I got good result when I tried the same code with a smaller dataset (100 instances and 5 features). Could someone please help me figure out what the problem could be? Thank you.