diff --git a/docs/cmd/kn_revision.md b/docs/cmd/kn_revision.md
index 01f393e23d..452fdf3125 100644
--- a/docs/cmd/kn_revision.md
+++ b/docs/cmd/kn_revision.md
@@ -6,6 +6,10 @@ Revision command group
 
 Revision command group
 
+```
+kn revision [flags]
+```
+
 ### Options
 
 ```
diff --git a/docs/cmd/kn_service.md b/docs/cmd/kn_service.md
index 85cd324ac9..5b4a36f95c 100644
--- a/docs/cmd/kn_service.md
+++ b/docs/cmd/kn_service.md
@@ -6,6 +6,10 @@ Service command group
 
 Service command group
 
+```
+kn service [flags]
+```
+
 ### Options
 
 ```
diff --git a/pkg/kn/core/root.go b/pkg/kn/core/root.go
index 7522af0255..cf5b812743 100644
--- a/pkg/kn/core/root.go
+++ b/pkg/kn/core/root.go
@@ -15,6 +15,7 @@
 package core
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"os"
@@ -71,11 +72,36 @@ Eventing: Manage event subscriptions and channels. Connect up event sources.`,
 	rootCmd.AddCommand(commands.NewCompletionCommand(p))
 	rootCmd.AddCommand(commands.NewVersionCommand(p))
 
+	// Deal with empty and unknown sub command groups
+	emptyAndUnknownSubcommands(rootCmd)
+
 	// For glog parse error.
 	flag.CommandLine.Parse([]string{})
 	return rootCmd
 }
 
+// emptyAndUnknownSubcommands adds a RunE to all commands that are groups to
+// deal with errors when called with empty or unknown sub command
+func emptyAndUnknownSubcommands(cmd *cobra.Command) {
+	for _, childCmd := range cmd.Commands() {
+		if childCmd.HasSubCommands() && childCmd.RunE == nil {
+			childCmd.RunE = func(aCmd *cobra.Command, args []string) error {
+				aCmd.Help()
+				fmt.Println()
+
+				if len(args) == 0 {
+					return errors.New(fmt.Sprintf("please provide a valid sub-command for \"kn %s\"", aCmd.Name()))
+				} else {
+					return errors.New(fmt.Sprintf("unknown sub-command \"%s\" for \"kn %s\"", args[0], aCmd.Name()))
+				}
+			}
+
+			// recurse to deal with child commands that are themselves command groups
+			emptyAndUnknownSubcommands(childCmd)
+		}
+	}
+}
+
 // initConfig reads in config file and ENV variables if set.
 func initConfig() {
 	if commands.CfgFile != "" {