diff --git a/usr.sbin/bhyve/mem.c b/usr.sbin/bhyve/mem.c
--- a/usr.sbin/bhyve/mem.c
+++ b/usr.sbin/bhyve/mem.c
@@ -45,6 +45,7 @@
 #include <stdlib.h>
 #include <vmmapi.h>
 
+#include "debug.h"
 #include "mem.h"
 
 struct mmio_rb_range {
@@ -163,6 +164,35 @@
 	return (error);
 }
 
+static int
+no_mem_handler(struct vcpu *vcpu __unused, int dir, uint64_t addr __unused,
+    int size, uint64_t *val, void *arg1 __unused, long arg2 __unused)
+{
+	if (dir == MEM_F_READ) {
+		switch (size) {
+		case 1:
+			*val = 0xff;
+			break;
+		case 2:
+			*val = 0xffff;
+			break;
+		case 4:
+			*val = 0xffffffff;
+			break;
+		case 8:
+			*val = 0xffffffffffffffff;
+			break;
+		}
+	}
+	return (0);
+}
+
+static struct mmio_rb_range fb_entry = {
+	.mr_param.handler = no_mem_handler,
+	.mr_param.base = 0,
+	.mr_param.size = 0xffffffffffffffff,
+};
+
 static int
 access_memory(struct vcpu *vcpu, uint64_t paddr, mem_cb_t *cb, void *arg)
 {
@@ -186,9 +216,10 @@
 			/* Update the per-vCPU cache */
 			mmio_hint[vcpuid] = entry;
 		} else if (mmio_rb_lookup(&mmio_rb_fallback, paddr, &entry)) {
-			perror = pthread_rwlock_unlock(&mmio_rwlock);
-			assert(perror == 0);
-			return (ESRCH);
+			entry = &fb_entry;
+			EPRINTLN(
+			    "Emulating access to non-existent address to %#lx\n",
+			    paddr);
 		}
 	}