The FastSerializerGenerator calls Schema.getName() to create temporary variable names when iterating over some arrays and maps. If the element type is a union, then the name created by Schema.UnionSchema.getName() is not a valid Java identifier. The generated code causes compiler errors that look like this:

/tmp/generated16917366869434796488/com/linkedin/avro/fastserde/generated/serialization/AVRO_1_11/Record_GenericSerializer_1168413672.java:97: error: ']' expected
                        CharSequence union[null, string]0 = null;

This is only a problem starting with org.apache.avro:avro:1.12.0, which was released last month: apache/avro@ed2dbd4#diff-22eb7312aa7f632f0f9b1c3f40de619f91a303ee39f43d3a62a338adf745d1efR1289 . (There are other test failures related to logical types that will need to be fixed for full Avro 1.12.0 compatibility, but this was the only issue causing compilation errors.)