java.lang.InternalError: Malformed class name in zeppelin paragraph

Question

I write a UDF and use it in sql in a spark paragraph like this:

import java.security.MessageDigest
spark.udf.register("covertMd5", (input: String) => {
     if (input == null || input.size == 0) null
     else {
         if (input.startsWith("_")) input
         else {
            var md5: MessageDigest = null
            try {
                md5 = MessageDigest.getInstance("MD5")
             }
            catch {
                case e: Exception => {
                e.printStackTrace
                println(e.getMessage)
                }
            }
            val byteArray: Array[Byte] = input.getBytes
            val md5Bytes: Array[Byte] = md5.digest(byteArray)
            var hexValue: String = ""
            var i: Integer = 0
            for (i <- 0 to md5Bytes.length - 1) {
                val str: Int = (md5Bytes(i).toInt) & 0xff
                //      println("str" + str)
                if (str < 16) {
                    hexValue = hexValue + "0"
                }
                hexValue = hexValue + (Integer.toHexString(str))
            }
            hexValue.toString
        }
     }
})

spark.sql("""select covertMd5(col) from table""")

This paragraph can be run normally about 10+ times, then it report error:

java.lang.InternalError: Malformed class name
  at java.lang.Class.getSimpleBinaryName(Class.java:1450)
  at java.lang.Class.getSimpleName(Class.java:1309)
  at org.apache.spark.sql.catalyst.expressions.ScalaUDF.udfErrorMessage$lzycompute(ScalaUDF.scala:1048)
  at org.apache.spark.sql.catalyst.expressions.ScalaUDF.udfErrorMessage(ScalaUDF.scala:1047)
  at org.apache.spark.sql.catalyst.expressions.ScalaUDF.doGenCode(ScalaUDF.scala:1000)
  at 
  ... 97 elided
Caused by: java.lang.StringIndexOutOfBoundsException: String index out of range: -83
  at java.lang.String.substring(String.java:1931)
  at java.lang.Class.getSimpleBinaryName(Class.java:1448)
  ... 164 more

If restart the zeppelin daemon, this problem can be fixed temporarily. But happened again after 10+ times execution.

Any one has idea? Thanks very much.