diff -r abf6ee4c477c -r 3bc26ffdf001 src/jdk.attach/macosx/classes/sun/tools/attach/VirtualMachineImpl.java
--- a/src/jdk.attach/macosx/classes/sun/tools/attach/VirtualMachineImpl.java	Thu Aug 22 10:43:25 2019 -0400
+++ b/src/jdk.attach/macosx/classes/sun/tools/attach/VirtualMachineImpl.java	Thu Aug 22 10:40:34 2019 -0700
@@ -59,8 +59,11 @@
         int pid;
         try {
             pid = Integer.parseInt(vmid);
+            if (pid < 1) {
+                throw new NumberFormatException();
+            }
         } catch (NumberFormatException x) {
-            throw new AttachNotSupportedException("Invalid process identifier");
+            throw new AttachNotSupportedException("Invalid process identifier: " + vmid);
         }
 
         // Find the socket file. If not found then we attempt to start the