HDFC Bank Ergo General Insurance — life insurance

My name is manisha vadhaiya widow of late ashwin vadhaiya. My husband had taken insurance of rs 49, 72, 611 by paying single premium of rs. 4, 72, 611. But after my husband's death due to myocardial infarction, hdfc refused the insurance claim saying that he had died due to pre-existing illness which is false. If he was ill at the time of insurance why did they take his insurance. They could have refused it at that time itself.
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