diff --git a/src/errors.h b/src/errors.h
index 43baa6a..d1cf4b4 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3333,4 +3333,6 @@
 	INIT(= N_("E1301: String, Number, List or Blob required for argument %d"));
 EXTERN char e_script_variable_was_deleted[]
 	INIT(= N_("E1302: Script variable was deleted"));
+EXTERN char e_custom_list_completion_function_does_not_return_list_but_str[]
+	INIT(= N_("E1303: Custom list completion function does not return a List but a %s"));
 #endif
diff --git a/src/eval.c b/src/eval.c
index db8b4e6..f208ab1 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -842,6 +842,7 @@
  * Call Vim script function "func" and return the result as a List.
  * Uses "argv" and "argc" as call_func_retstr().
  * Returns NULL when there is something wrong.
+ * Gives an error when the returned value is not a list.
  */
     void *
 call_func_retlist(
@@ -856,6 +857,8 @@
 
     if (rettv.v_type != VAR_LIST)
     {
+	semsg(_(e_custom_list_completion_function_does_not_return_list_but_str),
+		vartype_name(rettv.v_type));
 	clear_tv(&rettv);
 	return NULL;
     }
diff --git a/src/testdir/test_usercommands.vim b/src/testdir/test_usercommands.vim
index 91d8bfd..8a41004 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -666,7 +666,7 @@
     return "a\nb\n"
   endfunc
   command -nargs=* -complete=customlist,T1 TCmd1
-  call feedkeys(":TCmd1 \<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_fails('call feedkeys(":TCmd1 \<C-A>\<C-B>\"\<CR>", "xt")', 'E1303: Custom list completion function does not return a List but a string')
   call assert_equal('"TCmd1 ', @:)
   delcommand TCmd1
   delfunc T1
@@ -675,7 +675,7 @@
     return {}
   endfunc
   command -nargs=* -complete=customlist,T2 TCmd2
-  call feedkeys(":TCmd2 \<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_fails('call feedkeys(":TCmd2 \<C-A>\<C-B>\"\<CR>", "xt")', 'E1303: Custom list completion function does not return a List but a dict')
   call assert_equal('"TCmd2 ', @:)
   delcommand TCmd2
   delfunc T2
diff --git a/src/version.c b/src/version.c
index cd340b5..2d9b3ad 100644
--- a/src/version.c
+++ b/src/version.c
@@ -704,6 +704,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    437,
+/**/
     436,
 /**/
     435,