diff --git a/src/p_mach.cpp b/src/p_mach.cpp
index 0c076804..e53e1f4c 100644
--- a/src/p_mach.cpp
+++ b/src/p_mach.cpp
@@ -1645,7 +1645,8 @@ tribool PackMachBase<T>::canUnpack()
 
     unsigned const ncmds = mhdri.ncmds;
     int headway = (int)mhdri.sizeofcmds;
-    if (!ncmds || MAX_N_CMDS < ncmds || file_size < headway) {
+    if (!ncmds || MAX_N_CMDS < ncmds || file_size < headway
+     ||  headway < 4*4 ) {
         char msg[80]; snprintf(msg, sizeof(msg),
             "bad Mach_header ncmds=%d  sizeofcmds=0x%x", ncmds, headway);
         throwCantUnpack(msg);