Cannot deserialize the current JSON object (e.g. {\"name\":\"value\"})

Question

I am getting an error in postman like below

Cannot deserialize the current JSON object (e.g. {\"name\":\"value\"}) into type 'System.Char' because the type requires a JSON primitive value (e.g. string, number, boolean, null) to deserialize correctly.
To fix this error either change the JSON to a JSON primitive value (e.g. string, number, boolean, null) or change the deserialized type so that it is a normal .NET type (e.g. not a primitive type like integer, not a collection type like an array or List<T>) that can be deserialized from a JSON object. JsonObjectAttribute can also be added to the type to force it to deserialize from a JSON object.
Path 'cardNumber', line 2, position 15."

I am passing a string value:

{
    "cardNumber": "{{strCardNumber}}"
}

The card number is a collection variable which is set in previous request in same collection. I also checked whether the cardnumber is string in previous request with a test.

pm.expect(pm.collectionVariables.get("strCardNumber")).to.be.a('string');

This test passed, so the cardnumber is a string but I am not sure why I am getting this error. Can someone please help me in this?