Why RNF5410 for undefined variable in prototype call?

Not a showstopper by any means, more out of morbid curiousity. RPGLE V5R4
(but may still exist at higher versions). Consider the example program:

d ExecCmd pr extpgm('QCMDEXC')
d CmdString 3000a const options(*varsize)
d CmdStringLen 15p 5 const
d CmdOption 3a const options(*nopass)
/free
command = 'ovrdbf file(myfile) tofile(loyd/myfile)';
execcmd(command:%len(%trim(command)));
*inlr = *on;
return;
/end-free
There is an intentional syntax errror: the variable "command" is not
defined. The gist is using a prototyped call. QCMDEXC and the command to
execute are arbitrary and serve as examples only.

When compiled, these messages are generated:
*RNF7030 The name or indicator COMMAND is not defined. [expected]
*RNF7503 Expression contains an operand that is not defined. [command=...]
*RNF7503 Expression contains an operand that is not defined. [execcmd(...]
*RNF5410 The prototype for the call is not defined. [Huh??]

Why does RNF5410 occur? The prototype definitely exists. Without it, these
compile messages occur:
*RNF7030 The name or indicator COMMAND is not defined. [expected]
*RNF7030 THe name or indicator EXECCMD is not defined. [expected]
*RNF7503 Expression contains an operand that is not defined. [command=...]
*RNF7503 Expression contains an operand that is not defined. [execcmd(...]

The RNF5410 is just out of left field to me. I'm curious as to the rationale
of this erroneous (in this instance) error. Is the compiler saying that
because an argument is not defined, it can't execute the prototyped call?
That makes more sense to me than saying the prototype does not exist.

--Loyd