diff --git a/src/SPIN/pair_spin_exchange_biquadratic.cpp b/src/SPIN/pair_spin_exchange_biquadratic.cpp
index 59b959f4cc..7cdd8d0c19 100644
--- a/src/SPIN/pair_spin_exchange_biquadratic.cpp
+++ b/src/SPIN/pair_spin_exchange_biquadratic.cpp
@@ -180,7 +180,7 @@ void *PairSpinExchangeBiquadratic::extract(const char *str, int &dim)
 {
   dim = 0;
   if (strcmp(str,"cut") == 0) return (void *) &cut_spin_exchange_global;
-  return NULL;
+  return nullptr;
 }
 
 /* ---------------------------------------------------------------------- */
@@ -576,19 +576,19 @@ void PairSpinExchangeBiquadratic::read_restart(FILE *fp)
   int me = comm->me;
   for (i = 1; i <= atom->ntypes; i++) {
     for (j = i; j <= atom->ntypes; j++) {
-      if (me == 0) utils::sfread(FLERR,&setflag[i][j],sizeof(int),1,fp,NULL,error);
+      if (me == 0) utils::sfread(FLERR,&setflag[i][j],sizeof(int),1,fp,nullptr,error);
       MPI_Bcast(&setflag[i][j],1,MPI_INT,0,world);
       if (setflag[i][j]) {
         if (me == 0) {
-          utils::sfread(FLERR,&J1_mag[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&J1_mech[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&J2[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&J3[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&K1_mag[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&K1_mech[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&K2[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&K3[i][j],sizeof(double),1,fp,NULL,error);
-          utils::sfread(FLERR,&cut_spin_exchange[i][j],sizeof(double),1,fp,NULL,error);
+          utils::sfread(FLERR,&J1_mag[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&J1_mech[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&J2[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&J3[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&K1_mag[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&K1_mech[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&K2[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&K3[i][j],sizeof(double),1,fp,nullptr,error);
+          utils::sfread(FLERR,&cut_spin_exchange[i][j],sizeof(double),1,fp,nullptr,error);
         }
         MPI_Bcast(&J1_mag[i][j],1,MPI_DOUBLE,0,world);
         MPI_Bcast(&J1_mech[i][j],1,MPI_DOUBLE,0,world);
@@ -624,10 +624,10 @@ void PairSpinExchangeBiquadratic::write_restart_settings(FILE *fp)
 void PairSpinExchangeBiquadratic::read_restart_settings(FILE *fp)
 {
   if (comm->me == 0) {
-    utils::sfread(FLERR,&cut_spin_exchange_global,sizeof(double),1,fp,NULL,error);
-    utils::sfread(FLERR,&e_offset,sizeof(int),1,fp,NULL,error);
-    utils::sfread(FLERR,&offset_flag,sizeof(int),1,fp,NULL,error);
-    utils::sfread(FLERR,&mix_flag,sizeof(int),1,fp,NULL,error);
+    utils::sfread(FLERR,&cut_spin_exchange_global,sizeof(double),1,fp,nullptr,error);
+    utils::sfread(FLERR,&e_offset,sizeof(int),1,fp,nullptr,error);
+    utils::sfread(FLERR,&offset_flag,sizeof(int),1,fp,nullptr,error);
+    utils::sfread(FLERR,&mix_flag,sizeof(int),1,fp,nullptr,error);
   }
   MPI_Bcast(&cut_spin_exchange_global,1,MPI_DOUBLE,0,world);
   MPI_Bcast(&e_offset,1,MPI_INT,0,world);