KeyError

KeyError is raised when accessing a hash key that doesn't exist using fetch() without a default value, or when using ENV.fetch() for missing environment variables.

Common causes

• Using Hash#fetch with missing key and no default
• Missing environment variable with ENV.fetch
• Destructuring hash with missing required key
• Using dig on nested hash with missing intermediate key

How to fix it

• Provide a default value: hash.fetch(:key, default)
• Use hash[:key] which returns nil for missing keys
• Check key existence: hash.key?(:key)
• Use fetch with block: hash.fetch(:key) { compute_default }
• Set environment variables or provide fallbacks

Example

# Error example
hash = { name: "Alice" }
hash.fetch(:age)  # => KeyError: key not found: :age

# Fix with default value
hash.fetch(:age, 0)  # => 0

# Fix with block
hash.fetch(:age) { |key| "Unknown #{key}" }  # => "Unknown age"

# Or use bracket notation
hash[:age]  # => nil (no error)

Related errors