Message Oracle PLS-00615 - type mismatch found at string between CASE operand and WHEN operands

ERRORtype mismatch found at string between CASE operand and WHEN operands
CAUSEIn a simple CASE statement or expression, the CASE operand and WHEN operands do not match in datatype, and it is unclear which implicit conversion is required to correct the mismatch.
ACTIONChange the CASE operand and WHEN operands so that their datatypes match. Consider using datatype conversion functions in the CASE operand and/or WHEN operands.