diff --git a/src/testdir/test_channel.py b/src/testdir/test_channel.py
index d8830c5..40a2043 100644
--- a/src/testdir/test_channel.py
+++ b/src/testdir/test_channel.py
@@ -93,6 +93,13 @@
                         print("sending: {}".format(cmd))
                         self.request.sendall(cmd.encode('utf-8'))
                         response = "ok"
+                    elif decoded[1] == 'eval-error':
+                        # Send an eval request that works but the result can't
+                        # be encoded.
+                        cmd = '["eval","function(\\"tr\\")", -3]'
+                        print("sending: {}".format(cmd))
+                        self.request.sendall(cmd.encode('utf-8'))
+                        response = "ok"
                     elif decoded[1] == 'eval-bad':
                         # Send an eval request missing the third argument.
                         cmd = '["eval","xxx"]'
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 94ad81c..2a56c0d 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -118,10 +118,15 @@
   sleep 10m
   call assert_equal([-2, 'ERROR'], ch_sendexpr(handle, 'eval-result'))
 
+  " Send an eval request that works but can't be encoded.
+  call assert_equal('ok', ch_sendexpr(handle, 'eval-error'))
+  sleep 10m
+  call assert_equal([-3, 'ERROR'], ch_sendexpr(handle, 'eval-result'))
+
   " Send a bad eval request. There will be no response.
   call assert_equal('ok', ch_sendexpr(handle, 'eval-bad'))
   sleep 10m
-  call assert_equal([-2, 'ERROR'], ch_sendexpr(handle, 'eval-result'))
+  call assert_equal([-3, 'ERROR'], ch_sendexpr(handle, 'eval-result'))
 
   " Send an expr request
   call assert_equal('ok', ch_sendexpr(handle, 'an expr'))
diff --git a/src/testdir/test_json.vim b/src/testdir/test_json.vim
index 05c3949..52cffc8 100644
--- a/src/testdir/test_json.vim
+++ b/src/testdir/test_json.vim
@@ -75,6 +75,9 @@
   call assert_fails('echo jsonencode(function("tr"))', 'E474:')
   call assert_fails('echo jsonencode([function("tr")])', 'E474:')
   call assert_fails('echo jsonencode({"key":v:none})', 'E474:')
+
+  silent! let res = jsonencode(function("tr"))
+  call assert_equal("", res)
 endfunc
 
 func Test_decode()