[erlang-questions] Don't understand the error message: call to local/imported function ... is illegal in guard

[Donald Steven]

This code works:

     AlphaNumericChar = isAlphaNumeric(C),
     if AlphaNumericChar ->

but this code (which is functionally identical):

     if isAlphaNumeric(C) ->

produces an error message: call to local/imported function 
isAlphaNumeric/1 is illegal in guard

Why?