diff --git a/lib/mock_redis/string_methods.rb b/lib/mock_redis/string_methods.rb
index bbf9db7..0ef08e7 100644
--- a/lib/mock_redis/string_methods.rb
+++ b/lib/mock_redis/string_methods.rb
@@ -247,6 +247,7 @@ def setbit(key, offset, value)
 
       str = data[key] || ''
 
+      offset = offset.to_i
       offset_of_byte = offset / 8
       offset_within_byte = offset % 8