I had the Problem of setting the value of a custom field in my code
and therefore validating the new (calculated) value.
The CustomField had the format Redmine::FieldFormat::IntFormat.
The new value was a true Integer: 1
But the value was refused with the message that it's not a Number!

The Reason is, that Redmine::FieldFormat::IntFormat validates the value with RegExp:

value =~ /^[+-]?\d+$/

which will not return true if value is not a String.
I understand, that "normally" values of custom fields will be set via forms
and so usually all values will be Strings.

But I think IntFormat's validation should accept real Integers also!