diff --git a/R/read_fs_mgh.R b/R/read_fs_mgh.R
index 84c2aae..cd707ad 100755
--- a/R/read_fs_mgh.R
+++ b/R/read_fs_mgh.R
@@ -77,7 +77,7 @@ read.fs.mgh <- function(filepath, is_gzipped = "AUTO", flatten = FALSE, with_hea
 
         blah = Mdc %*% D;
         M = matrix(rep(0, 16), nrow=4);
-        M[1:3,1:3] = blah;
+        M[1:3,1:3] = as.matrix(blah);
         M[4,1:4] = c(0,0,0,1);
         M[1:3,4] = Pxyz_0;
 
diff --git a/R/write_fs_mgh.R b/R/write_fs_mgh.R
index d29be9f..6ca7cc2 100644
--- a/R/write_fs_mgh.R
+++ b/R/write_fs_mgh.R
@@ -34,7 +34,7 @@ write.fs.mgh <- function(filepath, data, vox2ras_matrix = NULL, mr_params = c(0.
     } else {
         cat(sprintf("The class of the 'vox2ras_matrix' argument is '%s'.", class(vox2ras_matrix)));
         print(vox2ras_matrix);
-        if(class(vox2ras_matrix) != "matrix") {
+        if(! is.matrix(vox2ras_matrix)) {
             stop("The 'vox2ras_matrix' argument must be a matrix.");
         }