Oracle Message PLS-00384 - type mismatch found at string in UPDATEs SET clause

ERRORtype mismatch found at string in UPDATEs SET clause
CAUSEThe column to the left of the equal sign in the SET clause of an UPDATE statement does not match in datatype with the column, expression, or subquery to the right of the equal sign, and it is unclear which implicit conversion is required to correct the mismatch.
ACTIONChange the expressions so that their datatypes match. You might want to use datatype conversion functions in the SET clause.