diff --git a/src/fix_external.cpp b/src/fix_external.cpp
index c9d019d963..349558fe86 100644
--- a/src/fix_external.cpp
+++ b/src/fix_external.cpp
@@ -65,6 +65,7 @@ FixExternal::FixExternal(LAMMPS *lmp, int narg, char **arg) :
   atom->add_callback(Atom::GROW);
 
   user_energy = 0.0;
+  memset(user_virial, 0, sizeof(user_virial));
 
   // optional vector of values provided by caller
   // vector_flag and size_vector are setup via set_vector_length()
@@ -321,6 +322,7 @@ double FixExternal::memory_usage()
 void FixExternal::grow_arrays(int nmax)
 {
   memory->grow(fexternal,nmax,3,"external:fexternal");
+  memset(&fexternal[0][0], 0, sizeof(double)*3*nmax);
 }
 
 /* ----------------------------------------------------------------------