--- a/src/java.net.http/share/classes/jdk/internal/net/http/common/Log.java	Thu Mar 29 09:55:50 2018 +0100
+++ b/src/java.net.http/share/classes/jdk/internal/net/http/common/Log.java	Thu Mar 29 09:59:24 2018 +0100
@@ -294,15 +294,25 @@
         if (headers != null) {
             Map<String,List<String>> h = headers.map();
             Set<Map.Entry<String,List<String>>> entries = h.entrySet();
+            String sep = "";
             for (Map.Entry<String,List<String>> entry : entries) {
                 String key = entry.getKey();
                 List<String> values = entry.getValue();
-                sb.append(prefix).append(key).append(":");
+                if (values == null || values.isEmpty()) {
+                    // should not happen
+                    sb.append(sep);
+                    sb.append(prefix).append(key).append(':');
+                    sep = "\n";
+                    continue;
+                }
                 for (String value : values) {
+                    sb.append(sep);
+                    sb.append(prefix).append(key).append(':');
                     sb.append(' ').append(value);
+                    sep = "\n";
                 }
-                sb.append('\n');
             }
+            sb.append('\n');
         }
     }