TypeError: no implicit conversion of nil into String Ruby / Rails Origin: Application & API OSI layer 7 · Application Last updated 5 Nov 2026

Ruby: TypeError: no implicit conversion of nil into String

A String method (+, File.join, Array#join usage) received nil instead of a string.

Exact message you may see TypeError: no implicit conversion of nil into StringTypeError (no implicit conversion of nil into String)no implicit conversion of Integer into Stringno implicit conversion of Symbol into StringTypeError: no implicit conversion from nil to integer
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Seen on: Ruby

Meaning

Typically an env var or attribute is nil (ENV["X"] + "/path", "a" + user.name). Use string interpolation ("#{x}"), to_s, or ENV.fetch("X") to fail clearly. The backtrace line shows the operation.

Common causes

  • ENV["VAR"] unset
  • nil attribute concatenated with +
  • File.join with nil segment
  • Missing default in config
  • Optional value from params

⚡ Quick fix

  1. ENV.fetch("VAR") to fail early or ENV.fetch("VAR", "default")
  2. Use interpolation "#{name}" or name.to_s
  3. Guard against nil with ||

Detailed fix by platform

Ruby

  1. path = File.join(ENV.fetch("APP_ROOT"), "tmp")

How to diagnose

  1. Value — Nil where?
  2. ENV — Set?
  3. Operation — Concatenation?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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