Issue #7539 has been updated by marcandre (Marc-Andre Lafortune). You'll get this error message for every failed conversion; the use the corresponding implicit conversion methods (`to_str`, `to_int`, ...) "" + 42 [].concat(1..2) I'm surprised this leads to confusion, but we could change the message to something like: "no implicit conversion of nil into String" ---------------------------------------- Bug #7539: Misleading error message "can't convert nil into string" https://bugs.ruby-lang.org/issues/7539#change-34590 Author: connec (Chris Connelly) Status: Open Priority: Normal Assignee: Category: core Target version: 1.9.3 ruby -v: ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux] When trying to call `String#+` with `nil` as an argument, you get the error "can't convert nil into String", which does not make sense (in fact seeming blatantly false) as `nil.to_s`, `String(nil)` etc. all return `''` without errors. Ideally, this method should use `to_s` to convert the argument, or else report an error along the lines of "can't append nil to string". Minimal test case: Actual: > '' + nil TypeError: can't convert nil into String Expected: > '' + nil '' -- http://bugs.ruby-lang.org/