"contrasts apply only to factors" in post-hoc analysis afer lmer fitting

Hi everyone,

I am applying  generalized linear mixed model to RNA-seq data under
quasi-poisson distribution. Here is the an example:

ts.da

   value tp tpf  treat cellline obs_effect
1     73  6  T6   ETOH        A          1
2     54  6  T6   ETOH        B          2
3     62 12 T12   ETOH        A          3
4     54 12 T12   ETOH        B          4
5     54 24 T24   ETOH        A          5
6     56 24 T24   ETOH        B          6
7     31 48 T48   ETOH        A          7
8     82 48 T48   ETOH        B          8
9     72  6  T6 shMYCN        C          9
10    95  6  T6 shMYCN        D         10
11    64 12 T12 shMYCN        C         11
12    90 12 T12 shMYCN        D         12
13    51 24 T24 shMYCN        C         13
14    64 24 T24 shMYCN        D         14
15    46 48 T48 shMYCN        C         15
16    63 48 T48 shMYCN        D         16

The specified model worked fine:
possionmix.full <- lmer(value ~ tpf * treat + (1|cellline)

,data=ts.da, family=poisson)

Then I tried to to post-hoc analysis with "phia":

testInteractions(possionmix.full)

Error in `contrasts<-`(`*tmp*`, value = "contr.treatment") : contrasts
apply only to factors

I am pretty sure that tpf and treat are factors:

ts.da$tpf

 [1] T6  T6  T12 T12 T24 T24 T48 T48 T6  T6  T12 T12 T24 T24 T48 T48
Levels: T6 T12 T24 T48

ts.da$treat

 [1] ETOH   ETOH   ETOH   ETOH   ETOH   ETOH   ETOH   ETOH   shMYCN

[11] shMYCN shMYCN shMYCN shMYCN shMYCN shMYCN
Levels: ETOH shMYCN