diff --git a/Gopkg.lock b/Gopkg.lock
index 27de95555..7780e6a8b 100644
--- a/Gopkg.lock
+++ b/Gopkg.lock
@@ -225,14 +225,14 @@
   revision = "ca137eb4b4389c9bc6f1a6d887f056bf16c00510"
 
 [[projects]]
-  digest = "1:143aae8d04a6133eea9c6400b90a1f47ae1100b48a1636160aba861d1b26c5b2"
+  digest = "1:980fd2c6afd6c268284d46dd66671771184a4002f6d516492cc596d2ca003543"
   name = "github.com/hashicorp/go-plugin"
   packages = [
     ".",
     "internal/plugin",
   ]
   pruneopts = "NUT"
-  revision = "3f118e8ee104b6f22aeb12453fab56aed1356186"
+  revision = "a1bc61569a26c0f65865932c0d55743b0567c494"
 
 [[projects]]
   branch = "master"
diff --git a/Gopkg.toml b/Gopkg.toml
index 39874975f..3a2fb8c94 100644
--- a/Gopkg.toml
+++ b/Gopkg.toml
@@ -115,7 +115,7 @@
 
 [[constraint]]
   name = "github.com/hashicorp/go-plugin"
-  revision = "3f118e8ee104b6f22aeb12453fab56aed1356186"
+  revision = "a1bc61569a26c0f65865932c0d55743b0567c494"
 
 [[constraint]]
   name = "github.com/golang/protobuf"
diff --git a/vendor/github.com/hashicorp/go-plugin/client.go b/vendor/github.com/hashicorp/go-plugin/client.go
index 2a8b42fc3..bc56559c6 100644
--- a/vendor/github.com/hashicorp/go-plugin/client.go
+++ b/vendor/github.com/hashicorp/go-plugin/client.go
@@ -87,6 +87,10 @@ type Client struct {
 	// goroutines.
 	clientWaitGroup sync.WaitGroup
 
+	// stderrWaitGroup is used to prevent the command's Wait() function from
+	// being called before we've finished reading from the stderr pipe.
+	stderrWaitGroup sync.WaitGroup
+
 	// processKilled is used for testing only, to flag when the process was
 	// forcefully killed.
 	processKilled bool
@@ -590,6 +594,12 @@ func (c *Client) Start() (addr net.Addr, err error) {
 	// Create a context for when we kill
 	c.doneCtx, c.ctxCancel = context.WithCancel(context.Background())
 
+	// Start goroutine that logs the stderr
+	c.clientWaitGroup.Add(1)
+	c.stderrWaitGroup.Add(1)
+	// logStderr calls Done()
+	go c.logStderr(cmdStderr)
+
 	c.clientWaitGroup.Add(1)
 	go func() {
 		// ensure the context is cancelled when we're done
@@ -602,6 +612,10 @@ func (c *Client) Start() (addr net.Addr, err error) {
 		pid := c.process.Pid
 		path := cmd.Path
 
+		// wait to finish reading from stderr since the stderr pipe reader
+		// will be closed by the subsequent call to cmd.Wait().
+		c.stderrWaitGroup.Wait()
+
 		// Wait for the command to end.
 		err := cmd.Wait()
 
@@ -624,11 +638,6 @@ func (c *Client) Start() (addr net.Addr, err error) {
 		c.exited = true
 	}()
 
-	// Start goroutine that logs the stderr
-	c.clientWaitGroup.Add(1)
-	// logStderr calls Done()
-	go c.logStderr(cmdStderr)
-
 	// Start a goroutine that is going to be reading the lines
 	// out of stdout
 	linesCh := make(chan string)
@@ -936,6 +945,7 @@ var stdErrBufferSize = 64 * 1024
 
 func (c *Client) logStderr(r io.Reader) {
 	defer c.clientWaitGroup.Done()
+	defer c.stderrWaitGroup.Done()
 	l := c.logger.Named(filepath.Base(c.config.Cmd.Path))
 
 	reader := bufio.NewReaderSize(r, stdErrBufferSize)
@@ -973,7 +983,22 @@ func (c *Client) logStderr(r io.Reader) {
 		entry, err := parseJSON(line)
 		// If output is not JSON format, print directly to Debug
 		if err != nil {
-			l.Debug(string(line))
+			// Attempt to infer the desired log level from the commonly used
+			// string prefixes
+			switch line := string(line); {
+			case strings.HasPrefix(line, "[TRACE]"):
+				l.Trace(line)
+			case strings.HasPrefix(line, "[DEBUG]"):
+				l.Debug(line)
+			case strings.HasPrefix(line, "[INFO]"):
+				l.Info(line)
+			case strings.HasPrefix(line, "[WARN]"):
+				l.Warn(line)
+			case strings.HasPrefix(line, "[ERROR]"):
+				l.Error(line)
+			default:
+				l.Debug(line)
+			}
 		} else {
 			out := flattenKVPairs(entry.KVPairs)
 
@@ -989,6 +1014,11 @@ func (c *Client) logStderr(r io.Reader) {
 				l.Warn(entry.Message, out...)
 			case hclog.Error:
 				l.Error(entry.Message, out...)
+			default:
+				// if there was no log level, it's likely this is unexpected
+				// json from something other than hclog, and we should output
+				// it verbatim.
+				l.Debug(string(line))
 			}
 		}
 	}
diff --git a/vendor/github.com/hashicorp/go-plugin/server.go b/vendor/github.com/hashicorp/go-plugin/server.go
index fc9f05a9f..4c230e3ab 100644
--- a/vendor/github.com/hashicorp/go-plugin/server.go
+++ b/vendor/github.com/hashicorp/go-plugin/server.go
@@ -363,14 +363,34 @@ func serverListener() (net.Listener, error) {
 }
 
 func serverListener_tcp() (net.Listener, error) {
-	minPort, err := strconv.ParseInt(os.Getenv("PLUGIN_MIN_PORT"), 10, 32)
-	if err != nil {
-		return nil, err
+	envMinPort := os.Getenv("PLUGIN_MIN_PORT")
+	envMaxPort := os.Getenv("PLUGIN_MAX_PORT")
+
+	var minPort, maxPort int64
+	var err error
+
+	switch {
+	case len(envMinPort) == 0:
+		minPort = 0
+	default:
+		minPort, err = strconv.ParseInt(envMinPort, 10, 32)
+		if err != nil {
+			return nil, fmt.Errorf("Couldn't get value from PLUGIN_MIN_PORT: %v", err)
+		}
 	}
 
-	maxPort, err := strconv.ParseInt(os.Getenv("PLUGIN_MAX_PORT"), 10, 32)
-	if err != nil {
-		return nil, err
+	switch {
+	case len(envMaxPort) == 0:
+		maxPort = 0
+	default:
+		maxPort, err = strconv.ParseInt(envMaxPort, 10, 32)
+		if err != nil {
+			return nil, fmt.Errorf("Couldn't get value from PLUGIN_MAX_PORT: %v", err)
+		}
+	}
+
+	if minPort > maxPort {
+		return nil, fmt.Errorf("ENV_MIN_PORT value of %d is greater than PLUGIN_MAX_PORT value of %d", minPort, maxPort)
 	}
 
 	for port := minPort; port <= maxPort; port++ {