diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -116,7 +116,7 @@
 static void	sigqueue_start(void);
 static void	sigfastblock_setpend(struct thread *td, bool resched);
 static void	sig_handle_first_stop(struct thread *td, struct proc *p,
-    int sig, bool ext);
+    int sig);
 
 static uma_zone_t	ksiginfo_zone = NULL;
 const struct filterops sig_filtops = {
@@ -2373,9 +2373,8 @@
 		if (pt_attach_transparent &&
 		    (p->p_flag & P_TRACED) != 0 &&
 		    (p->p_flag2 & P2_PTRACE_FSTP) != 0) {
-			td->td_dbgflags |= TDB_FSTP;
 			PROC_SLOCK(p);
-			sig_handle_first_stop(td, p, sig, true);
+			sig_handle_first_stop(NULL, p, sig);
 			PROC_SUNLOCK(p);
 			return (0);
 		}
@@ -2838,11 +2837,10 @@
 }
 
 static void
-sig_handle_first_stop(struct thread *td, struct proc *p, int sig, bool ext)
+sig_handle_first_stop(struct thread *td, struct proc *p, int sig)
 {
-	if ((td->td_dbgflags & TDB_FSTP) == 0 &&
-	    ((p->p_flag2 & P2_PTRACE_FSTP) != 0 ||
-	    p->p_xthread != NULL))
+	if (td != NULL && (td->td_dbgflags & TDB_FSTP) == 0 &&
+	    ((p->p_flag2 & P2_PTRACE_FSTP) != 0 || p->p_xthread != NULL))
 		return;
 
 	p->p_xsig = sig;
@@ -2852,7 +2850,7 @@
 	 * If we are on sleepqueue already, let sleepqueue
 	 * code decide if it needs to go sleep after attach.
 	 */
-	if (ext || td->td_wchan == NULL)
+	if (td != NULL && td->td_wchan == NULL)
 		td->td_dbgflags &= ~TDB_FSTP;
 
 	p->p_flag2 &= ~P2_PTRACE_FSTP;
@@ -2920,7 +2918,7 @@
 			 * already set p_xthread, the current thread will get
 			 * a chance to report itself upon the next iteration.
 			 */
-			sig_handle_first_stop(td, p, sig, false);
+			sig_handle_first_stop(td, p, sig);
 
 			if ((td->td_dbgflags & TDB_STOPATFORK) != 0) {
 				td->td_dbgflags &= ~TDB_STOPATFORK;
diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -930,12 +930,10 @@
 	}
 
 	if (tid == 0) {
-		if ((p->p_flag & P_STOPPED_TRACE) != 0) {
-			KASSERT(p->p_xthread != NULL, ("NULL p_xthread"));
+		if ((p->p_flag & P_STOPPED_TRACE) != 0)
 			td2 = p->p_xthread;
-		} else {
+		if (td2 == NULL)
 			td2 = FIRST_THREAD_IN_PROC(p);
-		}
 		tid = td2->td_tid;
 	}
 
@@ -1322,16 +1320,19 @@
 
 		/*
 		 * Clear the pending event for the thread that just
-		 * reported its event (p_xthread).  This may not be
-		 * the thread passed to PT_CONTINUE, PT_STEP, etc. if
-		 * the debugger is resuming a different thread.
+		 * reported its event (p_xthread), if any.  This may
+		 * not be the thread passed to PT_CONTINUE, PT_STEP,
+		 * etc. if the debugger is resuming a different
+		 * thread.  There might be no reporting thread if
+		 * the process was just attached.
 		 *
 		 * Deliver any pending signal via the reporting thread.
 		 */
-		MPASS(p->p_xthread != NULL);
-		p->p_xthread->td_dbgflags &= ~TDB_XSIG;
-		p->p_xthread->td_xsig = data;
-		p->p_xthread = NULL;
+		if (p->p_xthread != NULL) {
+			p->p_xthread->td_dbgflags &= ~TDB_XSIG;
+			p->p_xthread->td_xsig = data;
+			p->p_xthread = NULL;
+		}
 		p->p_xsig = data;
 
 		/*