Snowflake INNER JOIN results in Error: NULL result in a Non-Nullable Column

I am using the following query to inner join two tables. The columns I join on, are both of type varchar.

However when I use the following query, it results in Error: NULL result in a Non-Nullable Column

"RAW"."DB_FOO"."table" is a standard table. where EXTERNALID is of type VARCHAR(16777216)

EXTERNALID is NOT DECLARED as a "VARCHAR NOT NULL" in "RAW"."DB_FOO"."table" definition. It is nullable.

"RAW"."DB_BAR"."view" is a view where OPP_ID is of type VARCHAR(16777216)

SELECT 
   a.EXTERNALID
  ,c.OPP_ID

FROM 
    "RAW"."DB_FOO"."table" AS a
JOIN 
    "RAW"."DB_BAR"."view" AS c
ON 
    a.EXTERNALID = c.OPP_ID

When I try to explicity cast the type to varchar the query executes/works

SELECT 
   a.column
  ,c.column

FROM 
    "RAW"."DB_FOO"."table" AS a
JOIN 
    "RAW"."DB_BAR"."view" AS c
ON 
    a.EXTERNALID::varchar = c.OPP_ID::varchar

I have no idea why is that happening. Any help is greatly appreciated.