Index: source/Plugins/Process/POSIX/ProcessPOSIX.cpp
===================================================================
--- source/Plugins/Process/POSIX/ProcessPOSIX.cpp
+++ source/Plugins/Process/POSIX/ProcessPOSIX.cpp
@@ -340,6 +340,11 @@
     {
         assert(m_monitor);
         m_exit_now = true;
+        if (GetID() == LLDB_INVALID_PROCESS_ID)
+        {
+            error.SetErrorString("invalid process id");
+            return error;
+        }
         if (!m_monitor->Kill())
         {
             error.SetErrorToErrno();
Index: source/Target/Process.cpp
===================================================================
--- source/Target/Process.cpp
+++ source/Target/Process.cpp
@@ -3388,14 +3388,13 @@
             else
             {
                 if (GetID() != LLDB_INVALID_PROCESS_ID)
-                {
                     SetID (LLDB_INVALID_PROCESS_ID);
-                    const char *error_string = error.AsCString();
-                    if (error_string == NULL)
-                        error_string = "attach failed";
 
-                    SetExitStatus(-1, error_string);
-                }
+                const char *error_string = error.AsCString();
+                if (error_string == NULL)
+                    error_string = "attach failed";
+
+                SetExitStatus(-1, error_string);
             }
         }
     }
Index: test/functionalities/process_attach/attach_denied/Makefile
===================================================================
--- /dev/null
+++ test/functionalities/process_attach/attach_denied/Makefile
@@ -0,0 +1,7 @@
+LEVEL = ../../../make
+
+C_SOURCES := main.c
+
+EXE := AttachDenied
+
+include $(LEVEL)/Makefile.rules
Index: test/functionalities/process_attach/attach_denied/TestAttachDenied.py
===================================================================
--- /dev/null
+++ test/functionalities/process_attach/attach_denied/TestAttachDenied.py
@@ -0,0 +1,36 @@
+"""
+Test denied process attach.
+"""
+
+import os, time
+import unittest2
+import lldb
+from lldbtest import *
+import lldbutil
+
+exe_name = 'AttachDenied'  # Must match Makefile
+
+class AttachDeniedTestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    def test_attach_to_process_by_id_denied(self):
+        """Test attach by process id denied"""
+
+        self.buildDefault()
+        exe = os.path.join(os.getcwd(), exe_name)
+
+        # Spawn a new process
+        popen = self.spawnSubprocess(exe)
+        self.addTearDownHook(self.cleanupSubprocesses)
+
+        self.expect('process attach -p ' + str(popen.pid),
+                    startstr = 'error: attach failed:',
+                    error = True)
+
+
+if __name__ == '__main__':
+    import atexit
+    lldb.SBDebugger.Initialize()
+    atexit.register(lambda: lldb.SBDebugger.Terminate())
+    unittest2.main()
Index: test/functionalities/process_attach/attach_denied/main.c
===================================================================
--- /dev/null
+++ test/functionalities/process_attach/attach_denied/main.c
@@ -0,0 +1,38 @@
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+#if defined(__linux__)
+#include <sys/prctl.h>
+#elif defined(__APPLE__)
+#include <sys/types.h>
+#include <sys/ptrace.h>
+#endif
+
+int main(int argc, char const *argv[])
+{
+#if defined(__linux__)
+#if defined(PR_SET_DUMPABLE)
+    if (prctl(PR_SET_DUMPABLE, 0, 0, 0, 0) == -1)
+    {
+        char* err_msg = strerror(errno);
+        fprintf(stderr, "prctl(PR_SET_DUMPABLE) failed: %s\n", err_msg);
+        return -1;
+    }
+#endif
+#elif defined(__APPLE__)
+    if (ptrace(PT_DENY_ATTACH, 0, NULL, 0) == -1)
+    {
+        char* err_msg = strerror(errno);
+        fprintf(stderr, "ptrace(PT_DENY_ATTACH) failed: %s\n", err_msg);
+        return -1;
+    }
+#endif
+
+    // Waiting to be attempted to attach by the debugger.
+    sleep(30);
+
+    printf("Exiting now\n");
+    return 0;
+}