"Illegal variable context [DOM_VAR]" using mupad solver

I'm trying to symbolically solve a system of 51 equations in 51 unknowns. My code looks as follows:

equations:={eqn1,eqn2,...,eqn51}

unknowns:={var1,var2,...,var51}

solve(equations,unknowns)

Yet I keep getting the following error after evaluating solve:

Error: Illegal variable context [DOM_VAR]; during evaluation of 'solvelib::indets'

Any ideas why this is happening/how to get around it? Thanks!