diff -Naur fix_line_force.cpp new_fix_line_force.cpp 
--- fix_line_force.cpp	2008-01-09 16:56:57.000000000 -0500
+++ new_fix_line_force.cpp	2008-10-14 10:09:12.000000000 -0400
@@ -26,10 +26,17 @@
 FixLineForce::FixLineForce(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg)
 {
+  double rdir;
   if (narg != 6) error->all("Illegal fix lineforce command");
   xdir = atof(arg[3]);
   ydir = atof(arg[4]);
   zdir = atof(arg[5]);
+  rdir = sqrt(xdir*xdir + ydir*ydir + zdir*zdir);
+  if (rdir > 0.001) {
+    xdir = xdir/rdir; ydir = ydir/rdir; zdir = zdir/rdir;
+  } else {
+    error->all("In 'fix lineforce' command, the length of the direction vector is too small");
+  }
 }
 
diff -Naur fix_plane_force.cpp new_fix_plane_force.cpp
--- fix_plane_force.cpp	2008-01-09 16:56:57.000000000 -0500
+++ new_fix_plane_force.cpp	2008-10-14 10:10:17.000000000 -0400
@@ -26,10 +26,17 @@
 FixPlaneForce::FixPlaneForce(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg)
 {
+  double rdir;
   if (narg != 6) error->all("Illegal fix planeforce command");
   xdir = atof(arg[3]);
   ydir = atof(arg[4]);
   zdir = atof(arg[5]);
+  rdir = sqrt(xdir*xdir + ydir*ydir + zdir*zdir);
+  if (rdir > 0.001) {
+    xdir = xdir/rdir; ydir = ydir/rdir; zdir = zdir/rdir;
+  } else {
+    error->all("In 'fix lineforce' command, the length of the direction vector is too small");
+  }
 }