diff --git a/src/apps/snmp/snmp_scalar.c b/src/apps/snmp/snmp_scalar.c
index 49371039..094f52fe 100644
--- a/src/apps/snmp/snmp_scalar.c
+++ b/src/apps/snmp/snmp_scalar.c
@@ -193,7 +193,7 @@ snmp_scalar_array_get_next_instance(const u32_t *root_oid, u8_t root_oid_len, st
 static s16_t
 snmp_scalar_array_get_value(struct snmp_node_instance *instance, void *value)
 {
-  s16_t result = SNMP_ERR_NOSUCHINSTANCE;
+  s16_t result = -1;
   const struct snmp_scalar_array_node *array_node = (const struct snmp_scalar_array_node *)(const void *)instance->node;
   const struct snmp_scalar_array_node_def *array_node_def = (const struct snmp_scalar_array_node_def *)instance->reference.const_ptr;
 
@@ -206,7 +206,7 @@ snmp_scalar_array_get_value(struct snmp_node_instance *instance, void *value)
 static snmp_err_t
 snmp_scalar_array_set_test(struct snmp_node_instance *instance, u16_t value_len, void *value)
 {
-  snmp_err_t result = SNMP_ERR_NOSUCHINSTANCE;
+  snmp_err_t result = SNMP_ERR_NOTWRITABLE;
   const struct snmp_scalar_array_node *array_node = (const struct snmp_scalar_array_node *)(const void *)instance->node;
   const struct snmp_scalar_array_node_def *array_node_def = (const struct snmp_scalar_array_node_def *)instance->reference.const_ptr;
 
@@ -219,7 +219,7 @@ snmp_scalar_array_set_test(struct snmp_node_instance *instance, u16_t value_len,
 static snmp_err_t
 snmp_scalar_array_set_value(struct snmp_node_instance *instance, u16_t value_len, void *value)
 {
-  snmp_err_t result = SNMP_ERR_NOSUCHINSTANCE;
+  snmp_err_t result = SNMP_ERR_NOTWRITABLE;
   const struct snmp_scalar_array_node *array_node = (const struct snmp_scalar_array_node *)(const void *)instance->node;
   const struct snmp_scalar_array_node_def *array_node_def = (const struct snmp_scalar_array_node_def *)instance->reference.const_ptr;
 
diff --git a/src/apps/snmp/snmp_threadsync.c b/src/apps/snmp/snmp_threadsync.c
index cfcfb274..56d98862 100644
--- a/src/apps/snmp/snmp_threadsync.c
+++ b/src/apps/snmp/snmp_threadsync.c
@@ -55,7 +55,11 @@ threadsync_get_value_synced(void *ctx)
 {
   struct threadsync_data *call_data = (struct threadsync_data *)ctx;
 
-  call_data->retval.s16 = call_data->proxy_instance.get_value(&call_data->proxy_instance, call_data->arg1.value);
+  if (call_data->proxy_instance.get_value != NULL) {
+    call_data->retval.s16 = call_data->proxy_instance.get_value(&call_data->proxy_instance, call_data->arg1.value);
+  } else {
+    call_data->retval.s16 = -1;
+  }
 
   sys_sem_signal(&call_data->threadsync_node->instance->sem);
 }
@@ -76,7 +80,11 @@ threadsync_set_test_synced(void *ctx)
 {
   struct threadsync_data *call_data = (struct threadsync_data *)ctx;
 
-  call_data->retval.err = call_data->proxy_instance.set_test(&call_data->proxy_instance, call_data->arg2.len, call_data->arg1.value);
+  if (call_data->proxy_instance.set_test != NULL) {
+    call_data->retval.err = call_data->proxy_instance.set_test(&call_data->proxy_instance, call_data->arg2.len, call_data->arg1.value);
+  } else {
+    call_data->retval.err = SNMP_ERR_NOTWRITABLE;
+  }
 
   sys_sem_signal(&call_data->threadsync_node->instance->sem);
 }
@@ -98,7 +106,11 @@ threadsync_set_value_synced(void *ctx)
 {
   struct threadsync_data *call_data = (struct threadsync_data *)ctx;
 
-  call_data->retval.err = call_data->proxy_instance.set_value(&call_data->proxy_instance, call_data->arg2.len, call_data->arg1.value);
+  if (call_data->proxy_instance.set_value != NULL) {
+    call_data->retval.err = call_data->proxy_instance.set_value(&call_data->proxy_instance, call_data->arg2.len, call_data->arg1.value);
+  } else {
+    call_data->retval.err = SNMP_ERR_NOTWRITABLE;
+  }
 
   sys_sem_signal(&call_data->threadsync_node->instance->sem);
 }