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;
 
 		/*