diff --git a/R/calc_het.R b/R/calc_het.R
index 7cd09022..414ef72a 100644
--- a/R/calc_het.R
+++ b/R/calc_het.R
@@ -13,6 +13,14 @@
 #' Alternatively, this can be links to a set of cluster sockets, as
 #' produced by [parallel::makeCluster()].
 #'
+#' @details `calc_het()` looks at the genotype names (the 2nd
+#' dimension of the dimnames of the input `probs`), which must be
+#' two-letter names, and assumes that when the two letters are
+#' different it's a heterozygous genotype while if they're the
+#' same it's a homozygous genotype
+#'
+#' @seealso [calc_raw_het()], [calc_geno_freq()]
+#'
 #' @return
 #' The result is a vector of estimated heterozygosities
 #'
@@ -55,6 +63,11 @@ calc_het <-
     # determine which columns are het
     het_col <- vector("list", n_chr)
     geno <- dimnames(probs)[[2]]
+
+    if(any(nchar(geno) != 2)) {
+        stop("calc_het requires genotypes to have two-letter names")
+    }
+
     for(chr in seq_len(n_chr)) {
         a1 <- substr(geno[[chr]], 1, 1)
         a2 <- substr(geno[[chr]], 2, 2)
diff --git a/man/calc_het.Rd b/man/calc_het.Rd
index 1d1bfa15..22b8a533 100644
--- a/man/calc_het.Rd
+++ b/man/calc_het.Rd
@@ -25,6 +25,13 @@ The result is a vector of estimated heterozygosities
 \description{
 Calculate heterozygosites, by individual or by marker
 }
+\details{
+\code{calc_het()} looks at the genotype names (the 2nd
+dimension of the dimnames of the input \code{probs}), which must be
+two-letter names, and assumes that when the two letters are
+different it's a heterozygous genotype while if they're the
+same it's a homozygous genotype
+}
 \examples{
 iron <- read_cross2(system.file("extdata", "iron.zip", package="qtl2"))
 p <- calc_genoprob(iron, err=0.002)
@@ -36,3 +43,6 @@ het_ind <- calc_het(p)
 het_mar <- calc_het(p, "marker")
 
 }
+\seealso{
+\code{\link[=calc_raw_het]{calc_raw_het()}}, \code{\link[=calc_geno_freq]{calc_geno_freq()}}
+}