Names of the random effects should match eligible names given to
random in modeling functions. While with the random
argument to these functions, an
intercept is implicitly assumed, with randcov_params, an intercept must be
explicitly specified. That is, while with random, x | group
is shorthand for (1 | group) + (x | group), with randcov_params,
x | group implies just x | group, which means that if 1 | group
is also desired, it must be explicitly specified.