diff --git a/src/fix.cpp b/src/fix.cpp
index 2ec28be0a5..657d69a918 100644
--- a/src/fix.cpp
+++ b/src/fix.cpp
@@ -69,6 +69,7 @@ Fix::Fix(LAMMPS *lmp, int narg, char **arg) : Pointers(lmp)
 
   scalar_flag = vector_flag = array_flag = 0;
   peratom_flag = local_flag = 0;
+  size_array_rows_variable = 0;
 
   comm_forward = comm_reverse = comm_border = 0;
   restart_reset = 0;
diff --git a/src/fix.h b/src/fix.h
index 04920df238..7adb3a9b4b 100644
--- a/src/fix.h
+++ b/src/fix.h
@@ -57,6 +57,7 @@ class Fix : protected Pointers {
   int size_vector;               // length of global vector
   int size_array_rows;           // rows in global array
   int size_array_cols;           // columns in global array
+  int size_array_rows_variable;  // 1 if row count is unknown in advance
   int global_freq;               // frequency s/v data is available at
 
   int peratom_flag;              // 0/1 if per-atom data is stored
diff --git a/src/thermo.cpp b/src/thermo.cpp
index a3d14af479..b120642cf1 100644
--- a/src/thermo.cpp
+++ b/src/thermo.cpp
@@ -893,8 +893,11 @@ void Thermo::parse_fields(char *str)
         if (argindex1[nfield] > 0 && argindex2[nfield] > 0) {
           if (modify->fix[n]->array_flag == 0)
             error->all(FLERR,"Thermo fix does not compute array");
-          if (argindex1[nfield] > modify->fix[n]->size_array_rows ||
-              argindex2[nfield] > modify->fix[n]->size_array_cols)
+          if (argindex1[nfield] > modify->fix[n]->size_array_rows) {
+            if (modify->fix[n]->size_array_rows_variable == 0)
+              error->all(FLERR,"Thermo fix array is accessed out-of-range");
+          }
+          if (argindex2[nfield] > modify->fix[n]->size_array_cols)
             error->all(FLERR,"Thermo fix array is accessed out-of-range");
         }