TypeError: no implicit conversion in CI

Ruby refused to auto-convert one type into another for an operation. A string concatenation, array index, or similar got a value of an unexpected type.

What this error means

A test fails with TypeError naming the two types involved, such as "no implicit conversion of nil into String". The value feeding the operation is not what the code assumed.

TypeError:
       no implicit conversion of nil into String
     # ./app/helpers/path_helper.rb:5:in `+'

Common causes

nil where a String was expected

A value that was nil (an unset ENV, a missing key) was concatenated or interpolated into a String operation that does not accept nil.

Wrong type from a method

A method returned an Integer, Symbol, or Array where downstream code expected a String, and the operation will not coerce it implicitly.

Mixed types in an operation

Array#[] or similar received a non-Integer index, or arithmetic mixed incompatible types.

How to fix it

Convert explicitly or fix the source

1. Identify the value of the wrong type from the backtrace.
2. Either fix where it gets the wrong type, or convert explicitly with to_s, to_i, or to_a as appropriate.
3. Guard against nil with fetch or a default before the operation.

Coerce at the boundary

Normalize values to the expected type where they enter the code.

path = "/" + segment.to_s   # nil becomes "", no TypeError
index = key.to_i

How to prevent it

• Convert external values to the expected type at the boundary.
• Use fetch with defaults so nil never reaches type-sensitive operations.
• Add a test for the empty/nil input path.

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