Create a protobuf with

package book_sample;
message Book {
 optional string title = 1;
 optional string description = 2;
 optional int publicationYear = 3;
 }

It will show: Schema book.proto has errors

If you inspect the response you will see: cause: "Failed to resolve type of field "book_sample.Book.publicationYear". Type not found : int"

It is better to show what is the error