diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index f05ac15..4529a5e 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1225,6 +1225,7 @@
   call assert_fails("call byteidx([], 0)", 'E730:')
   call assert_fails("call byteidx('abc', [])", 'E745:')
   call assert_fails("call byteidx('abc', 0, {})", ['E728:', 'E728:'])
+  call assert_fails("call byteidx('abc', 0, -1)", ['E1023:', 'E1023:'])
 endfunc
 
 " Test for byteidxcomp() using a character index
@@ -1265,6 +1266,7 @@
   call assert_fails("call byteidxcomp([], 0)", 'E730:')
   call assert_fails("call byteidxcomp('abc', [])", 'E745:')
   call assert_fails("call byteidxcomp('abc', 0, {})", ['E728:', 'E728:'])
+  call assert_fails("call byteidxcomp('abc', 0, -1)", ['E1023:', 'E1023:'])
 endfunc
 
 " Test for byteidx() using a UTF-16 index
@@ -1625,7 +1627,6 @@
   " error cases
   call assert_equal(-1, utf16idx(test_null_string(), 0, v:true, v:true))
   call assert_fails('let l = utf16idx("ab", 0, v:false, [])', 'E1212:')
-  call assert_fails('echo strchars("", {})', ['E728:', 'E728:'])
 endfunc
 
 " Test for strutf16len()