diff --git a/src/USER-NETCDF/dump_netcdf.cpp b/src/USER-NETCDF/dump_netcdf.cpp
index 7156b773b3..a9532d1077 100644
--- a/src/USER-NETCDF/dump_netcdf.cpp
+++ b/src/USER-NETCDF/dump_netcdf.cpp
@@ -924,6 +924,8 @@ int DumpNetCDF::modify_param(int narg, char **arg)
     return 2;
   }
   else if (strcmp(arg[iarg],"at") == 0) {
+    if (!append_flag)
+      error->all(FLERR,"expected 'append yes' before 'at' keyword");
     iarg++;
     framei = force->inumeric(FLERR,arg[iarg]);
     if (framei < 0)  framei--;
diff --git a/src/USER-NETCDF/dump_netcdf_mpiio.cpp b/src/USER-NETCDF/dump_netcdf_mpiio.cpp
index 29c2b6cb1f..746b904655 100644
--- a/src/USER-NETCDF/dump_netcdf_mpiio.cpp
+++ b/src/USER-NETCDF/dump_netcdf_mpiio.cpp
@@ -920,6 +920,8 @@ int DumpNetCDFMPIIO::modify_param(int narg, char **arg)
     return 2;
   }
   else if (strcmp(arg[iarg],"at") == 0) {
+    if (!append_flag)
+      error->all(FLERR,"expected 'append yes' before 'at' keyword");
     iarg++;
     framei = force->inumeric(FLERR,arg[iarg]);
     if (framei < 0)  framei--;