From Longman Dictionary of Contemporary Englishcatch somebody by surprise, catch somebody off guard, catch somebody napping/unawarescatch somebody off guard, catch somebody napping/unawares catch somebody by surprisecatch somebody by surprise, catch somebody off guard, catch somebody napping/unawares (also catch somebody on the hop British English)EXPECT to do something or to happen when someone is not expecting it or prepared for it Her question caught him off guard. → catch
Examples from the Corpus
catch somebody by surprise, catch somebody off guard, catch somebody napping/unawares• My pregnancy caught us by surprise, but we're happy about it.• The public's reaction obviously caught the governor off guard. 