Error() model is singular - what does that mean

Hello

I have some test data that looks like that from a within subject experiment.
Subject   Task-Kind    Data-Kind   Time-Taken   Correct
1    A      Data1     5   1
1    A      Data1     3   0
1    A      Data1     1   1
1    A      Data2     8   1
1    A      Data2     7   0
1    A      Data2     5   0
1    A      Data3     2   1
1    A      Data3     7   0
1    A      Data3    5    0
1    A      Data3    6    0
1    B      Data1     3   1
1    B      Data1     1   1
1    B      Data1     3   0
1    B      Data2     9   0
1    B      Data2     8   1
1    B      Data2     5  0
1    B      Data3     2   1
1    B      Data3     7   2
1    B      Data3    5    3
1    B      Data3    6    0
1    C      Data1     3   1
1    C      Data1     1   1
1    C      Data1     3   0
1    C      Data2     9   0
1    C      Data2     8   1
1    C      Data2     5  0
1    C      Data3     2   1
1    C      Data3     7   2
1    C      Data3    5    3
1    C      Data3    6    0
2    A      Data1     5   1
2    A      Data1     3   0
2    A      Data1     1   1
2    A      Data2     8   1
2    A      Data2     7   0
2    A      Data2     5   0
2    A      Data3     2   1
2    A      Data3     7   0
2    A      Data3    5    0
2    A      Data3    6    0
2    B      Data1     3   1
2    B      Data1     1   1
2    B      Data1     3   0
2    B      Data2     9   0
2    B      Data2     8   1
2   B      Data2     5  0
2    B      Data3     2   1
2    B      Data3     7   2
2    B      Data3    5    3
2    B      Data3    6    0
2    C      Data1     3   1
2    C      Data1     1   1
2    C      Data1     3   0
2    C      Data2     9   0
2    C      Data2     8   1
2    C      Data2     5  0
2    C      Data3     2   1
2    C      Data3     7   2
2    C      Data3    5    3
2    C      Data3    6    0
.
.
.

some notes:
there are 20 subjects
there are 5 different kinds of tasks
There are 5 different kinds of data
and there are several different variations for a certain kind of task and
kind of data which is why for Subject = 1   Task-Kind=A  and Data-Kind=Data1 
we have 3 different results.

The measured parameters are time to complete the task and whether it was
correct or not (0 implies correct and 1 implies not correct)

I am computing the anova as follows:
aov.ex =
aov(Correct~Task-Kind*Data-Kind+Error(Subject/(Task-Kind*Data-Kind)),data=allDataRaw.xp)

since I want to see how the result is affected by the different kinds of
data as well as the the kind of task and I get a warning message saying:
Error() model is singular

I would be very grateful if someone could please tell me what does this
mean.
Thanks
Pascal

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