diff --git a/src/nvim/runtime.c b/src/nvim/runtime.c
index 9cbf140f11..9152f132ce 100644
--- a/src/nvim/runtime.c
+++ b/src/nvim/runtime.c
@@ -195,39 +195,44 @@ 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) {
-      size_t len = strlen(entry->es_name) + 15;
-      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;
       }
-      len += strlen(type_name);
-      ga_grow(&ga, (int)len);
       linenr_T lnum = idx == exestack.ga_len - 1
                       ? which == ESTACK_STACK ? SOURCING_LNUM : 0
                       : entry->es_lnum;
-      char *dots = idx == exestack.ga_len - 1 ? "" : "..";
-      if (lnum == 0) {
-        // 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.
-        vim_snprintf((char *)ga.ga_data + ga.ga_len, len, "%s%s%s",
-                     type_name, entry->es_name, dots);
-      } else {
-        vim_snprintf((char *)ga.ga_data + ga.ga_len, len, "%s%s[%" PRIdLINENR "]%s",
-                     type_name, entry->es_name, lnum, dots);
+
+      size_t len = es_name.size + type_name.size + 26;
+      ga_grow(&ga, (int)len);
+      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 += (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_len(&ga, S_LEN(".."));
       }
-      ga.ga_len += (int)strlen((char *)ga.ga_data + ga.ga_len);
     }
   }
 
+  // Only NUL-terminate when not returning NULL.
+  if (ga.ga_data != NULL) {
+    ga_append(&ga, NUL);
+  }
   return (char *)ga.ga_data;
 }
 
diff --git a/test/old/testdir/test_assert.vim b/test/old/testdir/test_assert.vim
index fa63af687d..03d3cab771 100644
--- a/test/old/testdir/test_assert.vim
+++ b/test/old/testdir/test_assert.vim
@@ -386,6 +386,19 @@ func Test_assert_fails_in_timer()
   call StopVimInTerminal(buf)
 endfunc
 
+" Check that a typed assert_equal() doesn't prepend an unnecessary ':'.
+func Test_assert_equal_typed()
+  let after =<< trim END
+    call feedkeys(":call assert_equal(0, 1)\<CR>", 't')
+    call feedkeys(":call writefile(v:errors, 'Xerrors')\<CR>", 't')
+    call feedkeys(":qa!\<CR>", 't')
+  END
+  call assert_equal(1, RunVim([], after, ''))
+  call assert_equal(['Expected 0 but got 1'], readfile('Xerrors'))
+
+  call delete('Xerrors')
+endfunc
+
 func Test_assert_beeps()
   new
   call assert_equal(0, assert_beeps('normal h'))