diff --git a/src/dump_atom.cpp b/src/dump_atom.cpp
index 1b3b0c2545..0d2586db35 100644
--- a/src/dump_atom.cpp
+++ b/src/dump_atom.cpp
@@ -162,7 +162,8 @@ void DumpAtom::write_data(int n, double *mybuf)
 
 void DumpAtom::header_binary(bigint ndump)
 {
-  fwrite(&update->ntimestep,sizeof(int),1,fp);
+  bigint ntimestep = update->ntimestep;
+  fwrite(&ntimestep,sizeof(bigint),1,fp);
   fwrite(&ndump,sizeof(bigint),1,fp);
   fwrite(&domain->triclinic,sizeof(int),1,fp);
   fwrite(&boxxlo,sizeof(double),1,fp);
@@ -182,7 +183,8 @@ void DumpAtom::header_binary(bigint ndump)
 
 void DumpAtom::header_binary_triclinic(bigint ndump)
 {
-  fwrite(&update->ntimestep,sizeof(int),1,fp);
+  bigint ntimestep = update->ntimestep;
+  fwrite(&ntimestep,sizeof(bigint),1,fp);
   fwrite(&ndump,sizeof(bigint),1,fp);
   fwrite(&domain->triclinic,sizeof(int),1,fp);
   fwrite(&boxxlo,sizeof(double),1,fp);
diff --git a/src/dump_custom.cpp b/src/dump_custom.cpp
index a2d9f36c13..33535ae73e 100644
--- a/src/dump_custom.cpp
+++ b/src/dump_custom.cpp
@@ -246,7 +246,8 @@ void DumpCustom::write_header(bigint ndump)
 
 void DumpCustom::header_binary(bigint ndump)
 {
-  fwrite(&update->ntimestep,sizeof(int),1,fp);
+  bigint ntimestep = update->ntimestep;
+  fwrite(&ntimestep,sizeof(int),1,fp);
   fwrite(&ndump,sizeof(bigint),1,fp);
   fwrite(&domain->triclinic,sizeof(int),1,fp);
   fwrite(&boxxlo,sizeof(double),1,fp);