diff --git a/src/core/ev.c b/src/core/ev.c
index c447ef96..5493f2bd 100644
--- a/src/core/ev.c
+++ b/src/core/ev.c
@@ -2472,12 +2472,12 @@ static JanetEVGenericMessage janet_go_thread_subr(JanetEVGenericMessage args) {
         }
 
         /* Get supervsior */
-        void *supervisor = NULL;
         if (flags & 0x8) {
             Janet sup =
                 janet_unmarshal(nextbytes, endbytes - nextbytes,
                                 JANET_MARSHAL_UNSAFE, NULL, &nextbytes);
-            supervisor = janet_unwrap_pointer(sup);
+            /* Hack - use a global variable to avoid longjmp clobber */
+            janet_vm.user = janet_unwrap_pointer(sup);
         }
 
         /* Set cfunction registry */
@@ -2506,17 +2506,32 @@ static JanetEVGenericMessage janet_go_thread_subr(JanetEVGenericMessage args) {
                                       JANET_MARSHAL_UNSAFE, NULL, &nextbytes);
         if (!janet_checktype(fiberv, JANET_FIBER)) janet_panicf("expected fiber, got %v", fiberv);
         JanetFiber *fiber = janet_unwrap_fiber(fiberv);
-        fiber->supervisor_channel = supervisor;
+        fiber->supervisor_channel = janet_vm.user;
         janet_schedule(fiber, value);
         janet_loop();
         args.tag = JANET_EV_TCTAG_NIL;
     } else {
-        if (janet_checktype(tstate.payload, JANET_STRING)) {
-            args.tag = JANET_EV_TCTAG_ERR_STRINGF;
-            args.argp = strdup((const char *) janet_unwrap_string(tstate.payload));
+        void *supervisor = janet_vm.user;
+        if (NULL != supervisor) {
+            /* Got a supervisor, write error there */
+            Janet pair[] = {
+                janet_ckeywordv("error"),
+                tstate.payload
+            };
+            janet_channel_push((JanetChannel *)supervisor,
+                    janet_wrap_tuple(janet_tuple_n(pair, 2)), 2);
+        } else if (flags & 0x1) {
+            /* No wait, just print to stderr */
+            janet_eprintf("thread start failure: %v\n", tstate.payload);
         } else {
-            args.tag = JANET_EV_TCTAG_ERR_STRING;
-            args.argp = "failed to start thread";
+            /* Make ev/thread call from parent thread error */
+            if (janet_checktype(tstate.payload, JANET_STRING)) {
+                args.tag = JANET_EV_TCTAG_ERR_STRINGF;
+                args.argp = strdup((const char *) janet_unwrap_string(tstate.payload));
+            } else {
+                args.tag = JANET_EV_TCTAG_ERR_STRING;
+                args.argp = "failed to start thread";
+            }
         }
     }
     janet_restore(&tstate);
@@ -2559,7 +2574,7 @@ JANET_CORE_FN(cfun_ev_thread,
         janet_marshal(buffer, janet_wrap_abstract(supervisor), NULL, JANET_MARSHAL_UNSAFE);
     }
     if (!(flags & 0x4)) {
-        janet_assert(janet_vm.registry_count <= UINT32_MAX, "assert failed size check");
+        janet_assert(janet_vm.registry_count <= INT32_MAX, "assert failed size check");
         uint32_t temp = (uint32_t) janet_vm.registry_count;
         janet_buffer_push_bytes(buffer, (uint8_t *) &temp, sizeof(temp));
         janet_buffer_push_bytes(buffer, (uint8_t *) janet_vm.registry, (int32_t) janet_vm.registry_count * sizeof(JanetCFunRegistry));
diff --git a/src/core/marsh.c b/src/core/marsh.c
index a9912190..47eff301 100644
--- a/src/core/marsh.c
+++ b/src/core/marsh.c
@@ -953,6 +953,7 @@ static const uint8_t *unmarshal_one_fiber(
     fiber->data = NULL;
     fiber->child = NULL;
     fiber->env = NULL;
+    fiber->last_value = janet_wrap_nil();
 #ifdef JANET_EV
     fiber->waiting = NULL;
     fiber->sched_id = 0;
@@ -1257,7 +1258,7 @@ static const uint8_t *unmarshal_one(
             data++;
             int32_t len = readnat(st, &data);
             if (len > 255) {
-                janet_panicf("invalid function");
+                janet_panicf("invalid function - too many environments (%d)", len);
             }
             func = janet_gcalloc(JANET_MEMORY_FUNCTION, sizeof(JanetFunction) +
                                  len * sizeof(JanetFuncEnv));
@@ -1265,7 +1266,8 @@ static const uint8_t *unmarshal_one(
             janet_v_push(st->lookup, *out);
             data = unmarshal_one_def(st, data, &def, flags + 1);
             if (def->environments_length != len) {
-                janet_panicf("invalid function");
+                janet_panicf("invalid function - env count does not match def (%d != %d)",
+                        len, def->environments_length);
             }
             func->def = def;
             for (int32_t i = 0; i < def->environments_length; i++) {
diff --git a/src/core/state.h b/src/core/state.h
index b6ee5f2f..b9106631 100644
--- a/src/core/state.h
+++ b/src/core/state.h
@@ -76,6 +76,9 @@ typedef struct {
 } JanetCFunRegistry;
 
 struct JanetVM {
+    /* Place for user data */
+    void *user;
+
     /* Top level dynamic bindings */
     JanetTable *top_dyns;
 
diff --git a/src/core/vm.c b/src/core/vm.c
index 27e200c1..75683a4e 100644
--- a/src/core/vm.c
+++ b/src/core/vm.c
@@ -1520,6 +1520,7 @@ int janet_init(void) {
     janet_vm.root_capacity = 0;
 
     /* Scratch memory */
+    janet_vm.user = NULL;
     janet_vm.scratch_mem = NULL;
     janet_vm.scratch_len = 0;
     janet_vm.scratch_cap = 0;
@@ -1579,6 +1580,7 @@ void janet_deinit(void) {
     janet_vm.abstract_registry = NULL;
     janet_vm.core_env = NULL;
     janet_vm.top_dyns = NULL;
+    janet_vm.user = NULL;
     janet_free(janet_vm.traversal_base);
     janet_vm.fiber = NULL;
     janet_vm.root_fiber = NULL;