IDL lambda: Attempt to subscript is out of range

idl 8.4 introduced the lambda function. I have IDL 8.6.1 and the lambda function does not fully work as documented:

IDL> f = lambda(x : x * x)
IDL> print, call_function(f, findgen(10))
      0.00000      1.00000      4.00000      9.00000      16.0000      25.0000      36.0000      49.0000      64.0000
      81.0000
IDL>  print, f(findgen(10))
IDL$LAMBDAF5 IDL$LAMBDAF5 IDL$LAMBDAF5 IDL$LAMBDAF5 IDL$LAMBDAF5 IDL$LAMBDAF5 IDL$LAMBDAF5 IDL$LAMBDAF5 IDL$LAMBDAF5
IDL$LAMBDAF5
IDL> print, f(5)
% Attempt to subscript F with <INT      (       5)> is out of range.

So, why f(5) does not return 25?

Solution

Oh, as commented here, this is documented.

Note: To make direct calls on a Lambda function, you should make sure that compile_opt strictarr (or compile_opt idl2) is turned on so that IDL interprets the parentheses as a function call instead of array indices. See COMPILE_OPT for details.

So compile_opt strictarr or compile_opt idl2.

However, I would have expected that direct calls are possible by default.