Error in MEEM when I include a new fixed effect

Question

I am unable to include more than 2 fixed effects in my LME without getting an error.

When I include a variable called DD in my fixed effects. I get an error. R recognizes my previous model - which is a simplified model without DD.

I know DD is the problem because when I take it off, the model works perfectly. But I know that for my clinical data DD is an important fixed effect that I need to account for. This is my model:

  mod1.lme <- lme(Variable1 ~ Age + Sex + DD + Category, 
random= ~1|Patient, data = AFN1, na.action = na.omit)
    summary(mod1.lme)

Error in MEEM(object, conLin, control$niterEM) :Singularity in backsolve at level 0, block 1

Data:

Patient Variable1  Age  Sex  DD  Category

   1a       12      30    0    346   A
   1b       22      30    0    346   F
   2a       29      67    0    700   A
   2b       12      67    0    700   A
   3a       50      19    1    36    N
   3b       54      19    1    36    N
   4a       44      36    1    55    F
   4b       22      36    1    55    A

Could this happen because Disease duration and another variable (maybe Age?) are colinear in my model? Could be LME warning me about the confounded fixed effects... I am not sure if this statement would mean that I can confidently drop DD from my model or would mean something a bit more serious.

I would really appreciate your help on this.

Best regards, Lili