diff --git a/toolbox/newfs_msdos.c b/toolbox/newfs_msdos.c
index 01517fd..5b98a01 100644
--- a/toolbox/newfs_msdos.c
+++ b/toolbox/newfs_msdos.c
@@ -798,6 +798,7 @@
                         __unused int oflag,struct bpb *bpb)
 {
     struct hd_geometry geom;
+    u_long block_size;
 
     if (ioctl(fd, BLKSSZGET, &bpb->bps)) {
         fprintf(stderr, "Error getting bytes / sector (%s)\n", strerror(errno));
@@ -806,11 +807,18 @@
 
     ckgeom(fname, bpb->bps, "bytes/sector");
 
-    if (ioctl(fd, BLKGETSIZE, &bpb->bsec)) {
+    if (ioctl(fd, BLKGETSIZE, &block_size)) {
         fprintf(stderr, "Error getting blocksize (%s)\n", strerror(errno));
         exit(1);
     }
 
+    if (block_size > UINT32_MAX) {
+        fprintf(stderr, "Error blocksize too large: %lu\n", block_size);
+        exit(1);
+    }
+
+    bpb->bsec = (u_int)block_size;
+
     if (ioctl(fd, HDIO_GETGEO, &geom)) {
         fprintf(stderr, "Error getting gemoetry (%s) - trying sane values\n", strerror(errno));
         geom.heads = 64;