diff --git a/syntax/stan.vim b/syntax/stan.vim
index f6fb7b5..812ce26 100644
--- a/syntax/stan.vim
+++ b/syntax/stan.vim
@@ -19,7 +19,7 @@ syntax match stanBlock "\vgenerated quantities"
 syntax match stanBlock "\vtransformed parameters"
 syntax match stanBlock "\vtransformed data"
 
-syntax keyword stanType int real vector simplex unit_vector ordered positive_ordered
+syntax keyword stanType int real vector simplex unit_vector ordered positive_ordered sum_to_zero_vector
 syntax keyword stanType row_vector matrix cholesky_factor_corr cholesky_factor_cov
 syntax keyword stanType corr_matrix cov_matrix
 syntax keyword stanType complex complex_matrix complex_vector complex_row_vector