Quick Answer

Invalid string indexing or manipulation

Understanding the Issue

This occurs when trying to access string indexes that don't exist or using invalid operations like [] with regex.

The Problem

This code demonstrates the issue:

Ruby Error 
# Problem: Invalid index
"hello"[10] # nil
"hello".index("z") # nil

# Problem: Invalid [] usage
"hello"[/z/] # IndexError

The Solution

Here's the corrected code:

Ruby Fixed 
# Solution: Check bounds/patterns
str = "hello"
str[1] if str.length > 1

# Safe regex match
if match = str.match(/e/)
  match[0]
end

Key Takeaways

Validate string bounds before access.