diff --git a/src/dump_atom.cpp b/src/dump_atom.cpp
index 79aaa1ffcd..dbb404b41b 100644
--- a/src/dump_atom.cpp
+++ b/src/dump_atom.cpp
@@ -562,7 +562,8 @@ void DumpAtom::write_binary(int n, double *mybuf)
 
 void DumpAtom::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/dump_cfg.cpp b/src/dump_cfg.cpp
index 28f5a35a4b..8e87f4104e 100644
--- a/src/dump_cfg.cpp
+++ b/src/dump_cfg.cpp
@@ -233,7 +233,8 @@ void DumpCFG::write_data(int n, double *mybuf)
 
 void DumpCFG::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/dump_custom.cpp b/src/dump_custom.cpp
index 16c67927bf..b2acdbfc51 100644
--- a/src/dump_custom.cpp
+++ b/src/dump_custom.cpp
@@ -1234,7 +1234,8 @@ void DumpCustom::write_binary(int n, double *mybuf)
 
 void DumpCustom::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/dump_local.cpp b/src/dump_local.cpp
index f9a970d2b2..96d8944e2f 100644
--- a/src/dump_local.cpp
+++ b/src/dump_local.cpp
@@ -399,7 +399,8 @@ void DumpLocal::write_data(int n, double *mybuf)
 
 void DumpLocal::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/dump_xyz.cpp b/src/dump_xyz.cpp
index ebbd432f5d..e88cea54b9 100644
--- a/src/dump_xyz.cpp
+++ b/src/dump_xyz.cpp
@@ -194,7 +194,8 @@ void DumpXYZ::write_data(int n, double *mybuf)
 
 void DumpXYZ::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */