diff --git a/cmd/jwt/README.md b/cmd/jwt/README.md
index 41eb89d..d9d06e7 100644
--- a/cmd/jwt/README.md
+++ b/cmd/jwt/README.md
@@ -8,6 +8,8 @@ The following will create and sign a token, then verify it and output the origin
 
      echo {\"foo\":\"bar\"} | ./jwt -key ../../test/sample_key -alg RS256 -sign - | ./jwt -key ../../test/sample_key.pub -alg RS256 -verify -
 
+Key files should be in PEM format. Other formats are not supported by this tool.
+
 To simply display a token, use:
 
     echo $JWT | ./jwt -show -