diff --git a/src/atom.cpp b/src/atom.cpp
index 4135298673..111ce7c93c 100644
--- a/src/atom.cpp
+++ b/src/atom.cpp
@@ -2471,7 +2471,7 @@ This function is called, e.g. from :doc:`fix property/atom <fix_property_atom>`.
  */
 int Atom::add_custom(const char *name, int flag, int cols)
 {
-  int index;
+  int index = -1;
 
   if ((flag == 0) && (cols == 0)) {
     index = nivector;
@@ -2511,7 +2511,8 @@ int Atom::add_custom(const char *name, int flag, int cols)
     dcols = (int *) memory->srealloc(dcols,ndarray*sizeof(int),"atom:dcols");
     dcols[index] = cols;
   }
-
+  if (index < 0)
+    error->all(FLERR,"Invalid call to Atom::add_custom()");
   return index;
 }