<< evaluation in Ruby

string_concat_eval.rb

def find(since=nil, til=nil)
  @where << since  ? " AND C.time >= #{since}" : ""
  @where << til ? " AND C.time <= #{til}" : ""

  find_by_sql make_sql #assume this generates SQL statement using @where and some other stuff...
end

# Question 1: Why does the above fail with this error:
# TypeError: can't convert nil into String
# 	from (irb):3:in `<<'
# 	from (irb):3
# 
# Question 2: How can we make this pass with the fewest code changes?
# Question 3: Is there another syntax we could use that would do the same thing?

The comments are bit light to read on a white background. Here they are:

Question 1: Why does the above fail with this error:

TypeError: can't convert nil into String
    from (irb):3:in `<<'
    from (irb):3

Question 2: How can we make this pass with the fewest code changes?
Question 3: Is there another syntax we could use that would do the same thing?