Index row size 2976 exceeds in postgresql

We have seen some of the indexes are getting failing with below error. ERROR: index row size 2976 exceeds byte version 4 maximum 2704 for index "idx1" HINT: Values larger than 1/3 of a buffer page cannot be indexed. Consider a function index of an MD5 hash of the value, or use full text indexing. Index : Create index idx1 on table1 using btree(upper(col1)); Table column length: 4000 pg_column_size for that column: 2931 Can you please suggest how to fix this issue