Hello colleagues, please help me understand the message, I got from Mathematica.

I have a set of points imported from a file. It is list of { x , y } coordinates.

uzly = Import["nodes.dat"];

uzly[[1]] (* coordinatws of node no 1 *)

{0., 0.}

Dimensions[uzly]

{56, 2}

Second dataset describes which nodes are connected into a elemet (a tringle)

prvky = Import["elements.dat"];

prvky[[1]] (* three nodes of element no 1 *)

{1, 5, 6}

Dimensions[prvky]

{78, 3}

I want to prepare polygons, that meansI want the three numbers in each element substitute with coordinates of the nodes with this numbers

I used a code

prvkyxy =

prvky /. {ni_, nj_, nk_} -> {uzly[[ni]], uzly[[nj]], uzly[[nk]]}

and the result seems to be OK:

prvkyxy[[1]] (* three {x y} coordinates of nodes of element no 1 *)

{{0., 0.}, {0.0111111, 0.}, {0.0111111, 0.0166667}}

Dimensions[prvkyxy]

{78, 3, 2}

But by applying the rule, I also got a message

Part::pspec: Part specification ni is neither a machine-sized integer nor a list of machine-sized integers. >>

Part::pspec: Part specification nj is neither a machine-sized integer nor a list of machine-sized integers. >>

Part::pspec: Part specification nk is neither a machine-sized integer nor a list of machine-sized integers. >>

General::stop: Further output of Part::pspec will be suppressed during this calculation. >>

which I cannot understand. The numbers are not out of bounds, the values of prvky are really integers and the result is what I am awaitng - so what is the problem? Why I got the message and what does it means?

Thanks!