Cayley Table Error Message

Question

I am trying to make a simple Cayley table for Z_5. Here's my attempt:

\[  
 \setlength{\extrarowheight}{3pt}  
    \begin{array}{l|*{5}{l}}  
    \oplus & [0] & [1] & [2] & [3] & [4] \\  
    \hline  
    [0] & [0] & [1] & [2] & [3] & [4] \\  
    [1] & [1] & [2] & [3] & [4] & [0] \\  
    [2] & [2] &[ 3] & [4] & [0] & [1] \\  
    [3] & [3] & [4] & [0] & [1] & [2] \\  
    [4] & [4] & [0] & [1] & [2] & [3] \\  
    \end{array}   
\]

I am getting the error message "Illegal unit of measure (pt inserted). If I insert other symbols in the third through sixth rows, such as powers of a, I don't get the error message. For instance,

\[  
    \setlength{\extrarowheight}{3pt}  
    \begin{array}{l|*{5}{l}}  
     \oplus & [0] & [1] & [2] & [3] & [4] \\  
    \hline  
    [0] & [0] & [1] & [2] & [3] & [4] \\  
    a   & a   & a^2 & a^3 & a^4  & a^5 \\  
    a^2 & a^2 & a^3 & a^4 & a^5  & a^6 \\  
    a^3 & a^3 & a^4 & a^5 & a^6  & a^7 \\  
    a^4 & a^4 & a^5 & a^6 & a^7  & a^8 \\  
    \end{array}  
\]