Substitution error for array element

[iron9light]

Hocon:

a {
  b: 0
  c: [
    "ccc"${a.b}
  ]
}

It will get exception:
Hocon.HoconParserException : Substitution value must match the rest of the field type or empty. Array value type: String, substitution type: Number. At path 'a.b', line 5, position -2.

Hocon below has no parsing exception:

a {
  b: 0
  c: "ccc"${a.b}
}

[holonsoftFounder]

AFAIK an array has to use a '='
Line 3 in your 1st example should be

c = [