diff --git a/cardano-cli/src/Cardano/CLI/Run/Key.hs b/cardano-cli/src/Cardano/CLI/Run/Key.hs
index 79e363dde8..901efcb27f 100644
--- a/cardano-cli/src/Cardano/CLI/Run/Key.hs
+++ b/cardano-cli/src/Cardano/CLI/Run/Key.hs
@@ -53,6 +53,7 @@ import           Cardano.Prelude (isSpace)
 
 import qualified Codec.Binary.Bech32 as Bech32
 import qualified Control.Exception as Exception
+import           Control.Monad (when)
 import           Data.Bifunctor (Bifunctor (..))
 import           Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
@@ -219,11 +220,23 @@ runGenerateMnemonicCmd
     , mnemonicWords
     } = do
     mnemonic <- firstExceptT KeyCmdMnemonicError $ generateMnemonic mnemonicWords
+    let expectedNumOfMnemonicWords = mnemonicSizeToInt mnemonicWords
+        obtainedNumOfMnemonicWords = length mnemonic
+    when (obtainedNumOfMnemonicWords /= expectedNumOfMnemonicWords) $
+      left $
+        KeyCmdWrongNumOfMnemonics expectedNumOfMnemonicWords obtainedNumOfMnemonicWords
     case mnemonicOutputFormat of
       Just outFile ->
         firstExceptT KeyCmdWriteFileError . newExceptT $
           writeTextFile outFile (T.unwords mnemonic)
       Nothing -> liftIO $ putStrLn $ T.unpack (T.unwords mnemonic)
+   where
+    mnemonicSizeToInt :: MnemonicSize -> Int
+    mnemonicSizeToInt MS12 = 12
+    mnemonicSizeToInt MS15 = 15
+    mnemonicSizeToInt MS18 = 18
+    mnemonicSizeToInt MS21 = 21
+    mnemonicSizeToInt MS24 = 24
 
 readExtendedVerificationKeyFile
   :: VerificationKeyFile In
diff --git a/cardano-cli/src/Cardano/CLI/Types/Errors/KeyCmdError.hs b/cardano-cli/src/Cardano/CLI/Types/Errors/KeyCmdError.hs
index 6bc8f1e5cd..1a41af1356 100644
--- a/cardano-cli/src/Cardano/CLI/Types/Errors/KeyCmdError.hs
+++ b/cardano-cli/src/Cardano/CLI/Types/Errors/KeyCmdError.hs
@@ -24,6 +24,11 @@ data KeyCmdError
   | KeyCmdReadMnemonicFileError !(FileError ())
   | KeyCmdWriteFileError !(FileError ())
   | KeyCmdMnemonicError MnemonicToSigningKeyError
+  | KeyCmdWrongNumOfMnemonics
+      !Int
+      -- ^ Expected number of mnemonics
+      !Int
+      -- ^ Actual number of mnemonics
   | KeyCmdByronKeyFailure !Byron.ByronKeyFailure
   | -- | Text representation of the parse error. Unfortunately, the actual
     -- error type isn't exported.
@@ -51,6 +56,8 @@ renderKeyCmdError err =
       prettyError fileErr
     KeyCmdMnemonicError mnemonicErr ->
       "Error converting the mnemonic into a key: " <> prettyError mnemonicErr
+    KeyCmdWrongNumOfMnemonics expected actual ->
+      "Internal error: expected " <> pretty expected <> " mnemonics, but got " <> pretty actual
     KeyCmdByronKeyFailure e ->
       Byron.renderByronKeyFailure e
     KeyCmdByronKeyParseError errTxt ->