diff --git a/src/nvim/runtime.c b/src/nvim/runtime.c
index 90f48333b2..9152f132ce 100644
--- a/src/nvim/runtime.c
+++ b/src/nvim/runtime.c
@@ -229,7 +229,10 @@ char *estack_sfile(estack_arg_T which)
     }
   }
 
-  ga_append(&ga, NUL);
+  // 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'))