Aggregation with group by clause

Facing some issues with the following case:

Json file (sample.json) is having the following content:

Query:

SELECT a, sum(b) FROM dfs.`C:\\Users\\user\\Desktop
sample.json` group by a;

Error:

UNSUPPORTED_OPERATION ERROR: Only COUNT, MIN and MAX aggregate functions supported for VarChar type

Observation:

If we query without using group by, then it is working fine without any error. If group by is used, then sum of null values is throwing the above error.

Can anyone please let us know the solution for this or if there are any alternative.