diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index ee509b98d4..67d7f83cb4 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -40,10 +40,12 @@ module ActiveRecord
     def merge!(other) # :nodoc:
       if other.is_a?(Hash)
         Relation::HashMerger.new(self, other).merge
+      elsif other.is_a?(Relation)
+        Relation::Merger.new(self, other).merge
       elsif other.respond_to?(:to_proc)
         instance_exec(&other)
       else
-        Relation::Merger.new(self, other).merge
+        raise ArgumentError, "#{other.inspect} is not an ActiveRecord::Relation"
       end
     end
 
diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index b58052a962..f46d414b95 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -24,10 +24,6 @@ module ActiveRecord
       def self.sanitize_sql_for_order(sql)
         sql
       end
-
-      def self.arel_table
-        Post.arel_table
-      end
     end
 
     def test_construction
@@ -239,6 +235,13 @@ module ActiveRecord
       assert_equal 3, relation.where(id: post.id).pluck(:id).size
     end
 
+    def test_merge_raises_with_invalid_argument
+      assert_raises ArgumentError do
+        relation = Relation.new(FakeKlass, :b, nil)
+        relation.merge(true)
+      end
+    end
+
     def test_respond_to_for_non_selected_element
       post = Post.select(:title).first
       assert_equal false, post.respond_to?(:body), "post should not respond_to?(:body) since invoking it raises exception"