Battleship -- simple code question

I’m banging my head over here, hopefully someone can quickly point out my mistake.
In trying to determine if the grid is empty, I defined two helper methods; empty_spaces and space_count. I structured the empty_spaces the exact same way that I structured my Board#count method (which passed the specs), I just wanted it to count something different.

However, now when I run it, I am getting an undefined method error;
NoMethodError:
undefined method `empty_spaces’ for [[nil, nil], [nil, nil]]:Array

It just doesn’t make sense to me. When I change the two methods so that they have the grid as the argument [ empty_spaces(grid) , space_count(grid) ], they work. My hypothesis is that because the methods are defined in the Board class, they aren’t working when called on Arrays(which the grid is). But again, I don’t understand why the Board#count worked and my Board#empty_spaces method didn’t, when they are structurally identical.

class Board

  attr_reader :grid

  def initialize(grid = nil)
    if grid == nil
      @grid = Board.default_grid
    else
      @grid = grid
    end
  end

  def self.default_grid
    row = [nil, nil, nil, nil, nil, nil, nil, nil, nil, nil]
    grid = []
    10.times {|i| grid << row}
    grid
  end

  def count
    targets = 0
    grid.each do |row|
       row.each {|space| targets += 1 if space == :s }
    end
    targets
  end

  def empty_spaces
    empty_space = 0
    grid.each do |row|
       row.each {|space| empty_space += 1 if space == nil }
    end
    empty_space
  end

  def space_count
    row_count = grid.length
    column_count = grid[0].length
    return (row_count * column_count)
  end

  def empty?(pos = nil)
    if pos == nil
      grid.each do |row|
         row.each { |space| return false if space != nil }
     end
    end

    if pos == nil
      return true if (grid.empty_spaces == grid.space_count)
    end

    row = pos.first
    col = pos.last
    return true if grid[row][col] == nil
    false
  end

NEVERMIND! I got it. I’m such a doofus.