MIPS: No error in Mars IDE, but error in QTSpim

I wrote my MIPS code in Mars, and it worked completely fine. But, I am having errors in QTSpim saying

Error: "Instruction references undefined symbol at 0x00400158 [0x00400158] 0x08000000 j 0x00000000 [next] ; 115: j next"

I have no idea what the error is telling. My instructor will test the code in QTSpim, so I have to make sure it works there.