diff --git a/command/kv/del/kv_delete.go b/command/kv/del/kv_delete.go
index 78e1967ee7..62fe49f7ca 100644
--- a/command/kv/del/kv_delete.go
+++ b/command/kv/del/kv_delete.go
@@ -78,7 +78,7 @@ func (c *cmd) Run(args []string) int {
 
 	// ModifyIndex is required for CAS
 	if c.cas && c.modifyIndex == 0 {
-		c.UI.Error("Must specify -modify-index with -cas!")
+		c.UI.Error("Cannot delete a key that does not exist. Provide a -modify-index greater than than 0.")
 		return 1
 	}
 
diff --git a/command/kv/del/kv_delete_test.go b/command/kv/del/kv_delete_test.go
index aeba734452..982a906853 100644
--- a/command/kv/del/kv_delete_test.go
+++ b/command/kv/del/kv_delete_test.go
@@ -32,7 +32,7 @@ func TestKVDeleteCommand_Validation(t *testing.T) {
 		},
 		"-cas no -modify-index": {
 			[]string{"-cas", "foo"},
-			"Must specify -modify-index",
+			"Cannot delete a key that does not exist",
 		},
 		"-modify-index no -cas": {
 			[]string{"-modify-index", "2", "foo"},
diff --git a/command/kv/put/kv_put.go b/command/kv/put/kv_put.go
index 0f416273d1..55c73439dc 100644
--- a/command/kv/put/kv_put.go
+++ b/command/kv/put/kv_put.go
@@ -99,12 +99,6 @@ func (c *cmd) Run(args []string) int {
 		return 1
 	}
 
-	// ModifyIndex is required for CAS
-	if c.cas && c.modifyIndex == 0 {
-		c.UI.Error("Must specify -modify-index with -cas!")
-		return 1
-	}
-
 	// Create and test the HTTP client
 	client, err := c.http.APIClient()
 	if err != nil {
@@ -127,6 +121,10 @@ func (c *cmd) Run(args []string) int {
 			c.UI.Error(fmt.Sprintf("Error! Did not write to %s: %s", key, err))
 			return 1
 		}
+		if !ok && c.modifyIndex == 0 {
+			c.UI.Error(fmt.Sprintf("Error! Did not write to %s: CAS performed with index=0 and key already exists.", key))
+			return 1
+		}
 		if !ok {
 			c.UI.Error(fmt.Sprintf("Error! Did not write to %s: CAS failed", key))
 			return 1
diff --git a/command/kv/put/kv_put_test.go b/command/kv/put/kv_put_test.go
index 12c99d6f37..eec776b4e5 100644
--- a/command/kv/put/kv_put_test.go
+++ b/command/kv/put/kv_put_test.go
@@ -39,10 +39,6 @@ func TestKVPutCommand_Validation(t *testing.T) {
 			[]string{"-release", "foo"},
 			"Missing -session",
 		},
-		"-cas no -modify-index": {
-			[]string{"-cas", "foo"},
-			"Must specify -modify-index",
-		},
 		"no key": {
 			[]string{},
 			"Missing KEY argument",