diff --git a/clientv3/op.go b/clientv3/op.go
index e2afdd52833..d3ed55b6274 100644
--- a/clientv3/op.go
+++ b/clientv3/op.go
@@ -215,14 +215,15 @@ func WithPrefix() OpOption {
 	}
 }
 
-// WithRange specifies the range of 'Get' or 'Delete' requests.
+// WithRange specifies the range of 'Get', 'Delete', 'Watch' requests.
 // For example, 'Get' requests with 'WithRange(end)' returns
 // the keys in the range [key, end).
+// endKey must be lexicographically greater than start key.
 func WithRange(endKey string) OpOption {
 	return func(op *Op) { op.end = []byte(endKey) }
 }
 
-// WithFromKey specifies the range of 'Get' or 'Delete' requests
+// WithFromKey specifies the range of 'Get', 'Delete', 'Watch' requests
 // to be equal or greater than the key in the argument.
 func WithFromKey() OpOption { return WithRange("\x00") }
 
diff --git a/etcdctl/README.md b/etcdctl/README.md
index aa4155d38b1..495acfcc630 100644
--- a/etcdctl/README.md
+++ b/etcdctl/README.md
@@ -221,7 +221,7 @@ OK
 
 ### WATCH [options] [key or prefix] [range_end]
 
-Watch watches events stream on keys or prefixes, [key or prefix, range_end) if `range-end` is given. The watch command runs until it encounters an error or is terminated by the user.
+Watch watches events stream on keys or prefixes, [key or prefix, range_end) if `range-end` is given. The watch command runs until it encounters an error or is terminated by the user.  If range_end is given, it must be lexicographically greater than key or "\x00".
 
 #### Options