diff --git a/sys/amd64/amd64/vm_machdep.c b/sys/amd64/amd64/vm_machdep.c
--- a/sys/amd64/amd64/vm_machdep.c
+++ b/sys/amd64/amd64/vm_machdep.c
@@ -157,6 +157,14 @@
 	/* Copy td1's pcb */
 	bcopy(td1->td_pcb, pcb2, sizeof(*pcb2));
 
+	/* But reset debug registers in the new thread */
+	pcb2->pcb_dr0 = 0;
+	pcb2->pcb_dr1 = 0;
+	pcb2->pcb_dr2 = 0;
+	pcb2->pcb_dr3 = 0;
+	pcb2->pcb_dr6 = 0;
+	pcb2->pcb_dr7 = 0;
+
 	/* Properly initialize pcb_save */
 	pcb2->pcb_save = get_pcb_user_save_pcb(pcb2);
 
diff --git a/sys/i386/i386/vm_machdep.c b/sys/i386/i386/vm_machdep.c
--- a/sys/i386/i386/vm_machdep.c
+++ b/sys/i386/i386/vm_machdep.c
@@ -158,6 +158,14 @@
 	/* Copy td1's pcb */
 	bcopy(td1->td_pcb, pcb2, sizeof(*pcb2));
 
+	/* But reset debug registers in the new thread */
+	pcb2->pcb_dr0 = 0;
+	pcb2->pcb_dr1 = 0;
+	pcb2->pcb_dr2 = 0;
+	pcb2->pcb_dr3 = 0;
+	pcb2->pcb_dr6 = 0;
+	pcb2->pcb_dr7 = 0;
+
 	/* Properly initialize pcb_save */
 	pcb2->pcb_save = get_pcb_user_save_pcb(pcb2);