patch 8.1.0019: error when defining a Lambda with index of a function result commit https://github.com/vim/vim/commit/b4518563c73460150344a57879bf5b22cb8b1c77 Author: Bram Moolenaar <Bram@vim.org> Date: Tue May 22 18:31:35 2018 +0200 patch 8.1.0019: error when defining a Lambda with index of a function result Problem: Error when defining a Lambda with index of a function result. Solution: When not evaluating an expression and skipping a function call, set the return value to VAR_UNKNOWN.

--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1349,8 +1349,16 @@ call_func(
     }
 
 
-    /* execute the function if no errors detected and executing */
-    if (evaluate && error == ERROR_NONE)
+    /*
+     * Execute the function if executing and no errors were detected.
+     */
+    if (!evaluate)
+    {
+	// Not evaluating, which means the return value is unknown.  This
+	// matters for giving error messages.
+	rettv->v_type = VAR_UNKNOWN;
+    }
+    else if (error == ERROR_NONE)
     {
 	char_u *rfname = fname;