diff --git a/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala b/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala
index 30976ac752..2340580b54 100644
--- a/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala
+++ b/core/src/main/scala/org/apache/spark/api/python/PythonWorkerFactory.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.api.python
 
-import java.io.{DataInputStream, DataOutputStream, InputStream, OutputStreamWriter}
+import java.io.{DataInputStream, DataOutputStream, EOFException, InputStream, OutputStreamWriter}
 import java.net.{InetAddress, ServerSocket, Socket, SocketException}
 import java.nio.charset.StandardCharsets
 import java.util.Arrays
@@ -182,7 +182,8 @@ private[spark] class PythonWorkerFactory(pythonExec: String, envVars: Map[String
 
       try {
         // Create and start the daemon
-        val pb = new ProcessBuilder(Arrays.asList(pythonExec, "-m", daemonModule))
+        val command = Arrays.asList(pythonExec, "-m", daemonModule)
+        val pb = new ProcessBuilder(command)
         val workerEnv = pb.environment()
         workerEnv.putAll(envVars.asJava)
         workerEnv.put("PYTHONPATH", pythonPath)
@@ -191,7 +192,29 @@ private[spark] class PythonWorkerFactory(pythonExec: String, envVars: Map[String
         daemon = pb.start()
 
         val in = new DataInputStream(daemon.getInputStream)
-        daemonPort = in.readInt()
+        try {
+          daemonPort = in.readInt()
+        } catch {
+          case _: EOFException =>
+            throw new SparkException(s"No port number in $daemonModule's stdout")
+        }
+
+        // test that the returned port number is within a valid range.
+        // note: this does not cover the case where the port number
+        // is arbitrary data but is also coincidentally within range
+        if (daemonPort < 1 || daemonPort > 0xffff) {
+          val exceptionMessage = f"""
+            |Bad data in $daemonModule's standard output. Invalid port number:
+            |  $daemonPort (0x$daemonPort%08x)
+            |Python command to execute the daemon was:
+            |  ${command.asScala.mkString(" ")}
+            |Check that you don't have any unexpected modules or libraries in
+            |your PYTHONPATH:
+            |  $pythonPath
+            |Also, check if you have a sitecustomize.py module in your python path,
+            |or in your python installation, that is printing to standard output"""
+          throw new SparkException(exceptionMessage.stripMargin)
+        }
 
         // Redirect daemon stdout and stderr
         redirectStreamsToStderr(in, daemon.getErrorStream)