diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 248edfc9cc..7d45787fae 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -7076,7 +7076,7 @@ static void f_assert_exception(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   rettv->vval.v_number = assert_exception(argvars);
 }
 
-/// "assert_fails(cmd [, error])" function
+/// "assert_fails(cmd [, error [, msg]])" function
 static void f_assert_fails(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   const char *const cmd = tv_get_string_chk(&argvars[0]);
@@ -7094,7 +7094,14 @@ static void f_assert_fails(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   if (!called_emsg) {
     prepare_assert_error(&ga);
     ga_concat(&ga, (const char_u *)"command did not fail: ");
-    ga_concat(&ga, (const char_u *)cmd);
+    if (argvars[1].v_type != VAR_UNKNOWN
+        && argvars[2].v_type != VAR_UNKNOWN) {
+      char *const tofree = encode_tv2echo(&argvars[2], NULL);
+      ga_concat(&ga, (char_u *)tofree);
+      xfree(tofree);
+    } else {
+      ga_concat(&ga, (const char_u *)cmd);
+    }
     assert_error(&ga);
     ga_clear(&ga);
     ret = 1;
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 8efbcc71f1..0ae250e626 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -30,7 +30,7 @@ return {
     assert_equal={args={2, 3}},
     assert_equalfile={args=2},
     assert_exception={args={1, 2}},
-    assert_fails={args={1, 2}},
+    assert_fails={args={1, 3}},
     assert_false={args={1, 2}},
     assert_inrange={args={3, 4}},
     assert_match={args={2, 3}},
diff --git a/test/functional/legacy/assert_spec.lua b/test/functional/legacy/assert_spec.lua
index 97c1f1405c..3cb5d97869 100644
--- a/test/functional/legacy/assert_spec.lua
+++ b/test/functional/legacy/assert_spec.lua
@@ -3,6 +3,7 @@ local nvim, call = helpers.meths, helpers.call
 local clear, eq = helpers.clear, helpers.eq
 local source, command = helpers.source, helpers.command
 local exc_exec = helpers.exc_exec
+local eval = helpers.eval
 
 local function expected_errors(errors)
   eq(errors, nvim.get_vvar('errors'))
@@ -233,20 +234,31 @@ describe('assert function:', function()
   -- assert_fails({cmd}, [, {error}])
   describe('assert_fails', function()
     it('should change v:errors when error does not match v:errmsg', function()
-      command([[call assert_fails('xxx', {})]])
+      eq(1, eval([[assert_fails('xxx', {})]]))
       command([[call assert_match("Expected {} but got 'E731:", v:errors[0])]])
       expected_errors({"Expected {} but got 'E731: using Dictionary as a String'"})
     end)
 
     it('should not change v:errors when cmd errors', function()
-      call('assert_fails', 'NonexistentCmd')
+      eq(0, eval([[assert_fails('NonexistentCmd')]]))
       expected_empty()
     end)
 
     it('should change v:errors when cmd succeeds', function()
-      call('assert_fails', 'call empty("")')
+      eq(1, eval([[assert_fails('call empty("")', '')]]))
       expected_errors({'command did not fail: call empty("")'})
     end)
+
+    it('can specify and get a message about what failed', function()
+      eq(1, eval([[assert_fails('xxx', {}, 'stupid')]]))
+      command([[call assert_match("stupid: Expected {} but got 'E731:", v:errors[0])]])
+      expected_errors({"stupid: Expected {} but got 'E731: using Dictionary as a String'"})
+    end)
+
+    it('can specify and get a message even when cmd succeeds', function()
+      eq(1, eval([[assert_fails('echo', '', 'echo command')]]))
+      expected_errors({'command did not fail: echo command'})
+    end)
   end)
 
   -- assert_inrange({lower}, {upper}, {actual}[, {msg}])