...neither slavery nor involuntary servitude should thereafter be introduced in the State except for the punishment of crimes; and that no male person or the age of twenty-one years, or female of the age of eighteen years, should be held to serve any person as a servant under any indenture thereafter made. It also rendered invalid any indenture thereafter made of any negro or mulatto where the term of service exceeded one year...
And Article 6, Section the Third with states:
Each and every person who has been bound to service by contract or indenture in virtue of the laws of Illinois territory heretofore existing, and in Conformity to the provisions of the same, without fraud or collusion, shall be held to a specific performance of their contracts or indentures; and such negroes and mulattoes as have registered in conformity with the aforesaid laws shall serve out the time appointed by said laws: Provided, however, that the children hereafter born of such persons, negroes and mulattoes, shall become free, the males at the age of twenty-one years; the females at the age of eighteen years.
This, however, was either ignored or overridden by the so-called "Black Laws" passed in 1819, one year after admission to the Union.
Evidence of the ignoring of the Constitution requirement is found in a letter from the then-Governor of Illinois Ninian Edwards. written to Col. A. G. S. Wright, a resident of Galena, Illinois dated Aug. 19, 1825, which states:
"I have just received your letter of the 4th-inst., and lose not a moment in replying to it.
"Whatever may have been the conceptions you had formed from my description, at Vandalia last winter, of the servants I have since sold you, I well know there was no intention on my part of deceiving you or any one else, and I should suppose your finding Charles so much better than you expected, sufficient to free me from any such suspicion, since, as he was capable of being the most valuable, if I had intended to deceive, I must have acted most strangely in representing him so much worse, and the others so, much better than they respectively deserved. The truth is, that I said nothing then, which I did not at that time, and which I do not now, believe to be true"...
..."I could have had no motive to deceive by any description I gave of those Servants, because I did not suppose anyone would have purchased them without seeing them and judging for himself."
And if the Governor of the state didn't particularly pay any attention to the Constitutional requirement, you can be sure that there were others and continued to be so for some time without interference by state officials, thus the recording of 747 slaves during the 1830 census.
Not surprising at all.
Doggone shame they had to adopt the same custom in Northern Illinois.