diff --git a/src/library.cpp b/src/library.cpp
index 793c817e9a..d045d08adc 100644
--- a/src/library.cpp
+++ b/src/library.cpp
@@ -4850,8 +4850,9 @@ int lammps_id_name(void *handle, const char *category, int idx, char *buffer, in
   auto lmp = (LAMMPS *) handle;
 
   if (strcmp(category,"compute") == 0) {
-    if ((idx >=0) && (idx < lmp->modify->ncompute)) {
-      strncpy(buffer, lmp->modify->compute[idx]->id, buf_size);
+    auto icompute = lmp->modify->get_compute_by_index(idx);
+    if (icompute) {
+      strncpy(buffer, icompute->id, buf_size);
       return 1;
     }
   } else if (strcmp(category,"dump") == 0) {
@@ -4860,8 +4861,9 @@ int lammps_id_name(void *handle, const char *category, int idx, char *buffer, in
       return 1;
     }
   } else if (strcmp(category,"fix") == 0) {
-    if ((idx >=0) && (idx < lmp->modify->nfix)) {
-      strncpy(buffer, lmp->modify->fix[idx]->id, buf_size);
+    auto ifix = lmp->modify->get_fix_by_index(idx);
+    if (ifix) {
+      strncpy(buffer, ifix->id, buf_size);
       return 1;
     }
   } else if (strcmp(category,"group") == 0) {
diff --git a/src/modify.h b/src/modify.h
index b339f31530..bd80cbece1 100644
--- a/src/modify.h
+++ b/src/modify.h
@@ -113,7 +113,9 @@ class Modify : protected Pointers {
   int find_fix(const std::string &);
   // new API
   Fix *get_fix_by_id(const std::string &) const;
-  Fix *get_fix_by_index(int idx) const { return fix[idx]; }
+  Fix *get_fix_by_index(int idx) const {
+    return ((idx >= 0) && (idx < nfix)) ? fix[idx] : nullptr;
+  }
   const std::vector<Fix *> get_fix_by_style(const std::string &) const;
   const std::vector<Fix *> &get_fix_list();
 
@@ -127,7 +129,9 @@ class Modify : protected Pointers {
   int find_compute(const std::string &);
   // new API
   Compute *get_compute_by_id(const std::string &) const;
-  Compute *get_compute_by_index(int idx) const { return compute[idx]; }
+  Compute *get_compute_by_index(int idx) const {
+    return ((idx >= 0) && (idx < nfix)) ? compute[idx] : nullptr;
+  }
   const std::vector<Compute *> get_compute_by_style(const std::string &) const;
   const std::vector<Compute *> &get_compute_list();