nullable-enum-contains-null

When a schema is marked as nullable, and also defines an enum array, the enum must explicitly contain a null value (not the string "null").

This rule validates both OpenAPI 3.0 (nullable: true) and OpenAPI 3.1 (type: [<type>, "null"]) nullable patterns.

Why did this violation appear?

A schema is marked as nullable but its enum array does not contain an actual null value. Simply marking a schema as nullable is not enough when using enums - the null value must be explicitly listed in the enum array.

Bad example

OpenAPI 3.0 - nullable enum without null:

components:
  schemas:
    Status:
      type: string
      nullable: true
      enum: [active, inactive, pending]
      # Missing null in enum array

OpenAPI 3.1 - nullable type enum without null:

components:
  schemas:
    Priority:
      type: [string, "null"]
      enum: [low, medium, high]
      # Missing null in enum array

Using the string "null" instead of actual null:

components:
  schemas:
    State:
      type: string
      nullable: true
      enum: [active, inactive, "null"]
      # String "null" is not the same as null

Good Example

OpenAPI 3.0 with explicit null in enum:

components:
  schemas:
    Status:
      type: string
      nullable: true
      enum: [active, inactive, pending, null]

OpenAPI 3.1 with explicit null in enum:

components:
  schemas:
    Priority:
      type: [string, "null"]
      enum: [low, medium, high, null]

Parameter with nullable enum:

paths:
  /items:
    get:
      parameters:
        - name: filter
          in: query
          schema:
            type: string
            nullable: true
            enum: [recent, popular, trending, null]

How do I fix this violation?

Add null (unquoted) to your enum array. Make sure it’s an actual null value, not the string "null".

Important: The null value should not be quoted. Use null, not "null".

# Correct
enum: [value1, value2, null]

# Incorrect
enum: [value1, value2, "null"]

This ensures that API consumers can properly send null values when the enum is used for input, and handle null values correctly when the enum is used for output.