diff --git a/src/nvim/runtime.c b/src/nvim/runtime.c
index 3127a36bde..90f48333b2 100644
--- a/src/nvim/runtime.c
+++ b/src/nvim/runtime.c
@@ -195,38 +195,41 @@ char *estack_sfile(estack_arg_T which)
   for (int idx = 0; idx < exestack.ga_len; idx++) {
     entry = ((estack_T *)exestack.ga_data) + idx;
     if (entry->es_name != NULL) {
-      char *type_name = "";
+      String type_name = STATIC_CSTR_AS_STRING("");
+      String es_name = cstr_as_string(entry->es_name);
       if (entry->es_type != last_type) {
         switch (entry->es_type) {
         case ETYPE_SCRIPT:
-          type_name = "script "; break;
+          type_name = STATIC_CSTR_AS_STRING("script "); break;
         case ETYPE_UFUNC:
-          type_name = "function "; break;
+          type_name = STATIC_CSTR_AS_STRING("function "); break;
         default:
-          type_name = ""; break;
+          break;
         }
         last_type = entry->es_type;
       }
       linenr_T lnum = idx == exestack.ga_len - 1
                       ? which == ESTACK_STACK ? SOURCING_LNUM : 0
                       : entry->es_lnum;
-      size_t len = strlen(entry->es_name) + strlen(type_name) + 26;
+
+      size_t len = es_name.size + type_name.size + 26;
       ga_grow(&ga, (int)len);
-      ga_concat(&ga, type_name);
-      ga_concat(&ga, entry->es_name);
+      ga_concat_len(&ga, type_name.data, type_name.size);
+      ga_concat_len(&ga, es_name.data, es_name.size);
       // For the bottom entry of <sfile>: do not add the line number, it is used in
       // <slnum>.  Also leave it out when the number is not set.
       if (lnum != 0) {
-        ga.ga_len += vim_snprintf((char *)ga.ga_data + ga.ga_len, 23,
-                                  "[%" PRIdLINENR "]", lnum);
+        ga.ga_len += (int)vim_snprintf_safelen((char *)ga.ga_data + ga.ga_len,
+                                               len - (size_t)ga.ga_len,
+                                               "[%" PRIdLINENR "]", lnum);
       }
       if (idx != exestack.ga_len - 1) {
-        ga_concat(&ga, "..");
+        ga_concat_len(&ga, S_LEN(".."));
       }
     }
   }
 
-  ga_append(&ga, '\0');
+  ga_append(&ga, NUL);
   return (char *)ga.ga_data;
 }