diff --git a/digiline.lua b/digiline.lua
index 194922c..b8ec8c5 100644
--- a/digiline.lua
+++ b/digiline.lua
@@ -32,7 +32,7 @@ ham_radio.digiline_effector = function(pos, _, channel, msg)
 
   elseif msg.command == "frequency" then
     local new_frequency = msg.value
-    if ham_radio.validate_frequency(new_frequency) then
+    if ham_radio.validate_frequency(new_frequency).result then
       meta:set_string("frequency", new_frequency)
       ham_radio.transmitter_update_infotext(meta)
       ham_radio.save_transmitter(pos, meta)  
diff --git a/helpers.lua b/helpers.lua
index d4cafd3..6e732ad 100644
--- a/helpers.lua
+++ b/helpers.lua
@@ -1,23 +1,33 @@
 function ham_radio.validate_frequency(frequency, is_receiver)
   if frequency == "" then
-    return true -- empty frequency is allowed to disable transmitter/receiver
+    return { result = true, message = '' } -- empty frequency is allowed to disable transmitter/receiver
   end
   local transmission_is_allowed = true
   local num_freq = tonumber(frequency)
   local freq = tostring(num_freq)
-  if is_receiver ~= nil and next(ham_radio.find_transmitters(frequency)) then
+  if is_receiver == nil and next(ham_radio.find_transmitters(frequency)) then
     if num_freq >= ham_radio.settings.locked_frequency.min
     and num_freq <= ham_radio.settings.locked_frequency.max then
       -- transmitter is in locked frequency range
       transmission_is_allowed = false
     end
   end
-  return freq == frequency
-    and num_freq ~= nil
-    and num_freq == math.floor(num_freq)
-    and num_freq >= ham_radio.settings.frequency.min
-    and num_freq <= ham_radio.settings.frequency.max
-    and transmission_is_allowed
+  local result = true
+  local message = ''
+  if freq ~= frequency or num_freq ~= math.floor(num_freq) then
+    result = false
+    message = 'Error: invalid frequency value.'
+  elseif num_freq == nil then
+    result = false
+    message = 'Error: frequency should be numeric.'
+  elseif num_freq < ham_radio.settings.frequency.min or num_freq > ham_radio.settings.frequency.max then
+    result = false
+    message = 'Error: frequency is out of range.'
+  elseif transmission_is_allowed == false then
+    result = false
+    message = 'Error: frequency is occupied by other transmitter.'
+  end
+  return { result = result, message = message }
 end
 
 function ham_radio.find_transmitters(frequency)
diff --git a/init.lua b/init.lua
index 1c65eb0..1254b9c 100644
--- a/init.lua
+++ b/init.lua
@@ -40,6 +40,10 @@ function ham_radio.play_tuning_sound(player)
   )
 end
 
+function ham_radio.errormsg(player, message)
+  minetest.chat_send_player(player:get_player_name(), minetest.colorize("#FCAD00", message))
+end
+
 dofile(modpath.."/config.lua")
 
 dofile(modpath.."/helpers.lua")
diff --git a/receiver.lua b/receiver.lua
index c4531f1..e09c056 100644
--- a/receiver.lua
+++ b/receiver.lua
@@ -37,7 +37,9 @@ minetest.register_on_player_receive_fields(function(player, formname, fields)
   if formname ~= "ham_radio:configure_handheld_receiver" or not minetest.is_player(player) then
     return false
   end
-  if not ham_radio.validate_frequency(fields.frequency, true) then
+  local is_frequency_valid = ham_radio.validate_frequency(fields.frequency, true)
+  if is_frequency_valid.result == false then
+    ham_radio.errormsg(player, is_frequency_valid.message)
     return false
   end
   local item = player:get_wielded_item()
diff --git a/transmitter.lua b/transmitter.lua
index e36a5cc..dd81c1b 100644
--- a/transmitter.lua
+++ b/transmitter.lua
@@ -62,11 +62,16 @@ minetest.register_node("ham_radio:transmitter", {
     if (
       fields.quit ~= "true"
       or minetest.is_protected(pos, sender:get_player_name()) 
-      or not ham_radio.validate_frequency(fields.frequency)
     ) then
       return
     end
 
+    local is_frequency_valid = ham_radio.validate_frequency(fields.frequency)
+    if is_frequency_valid.result == false then
+      ham_radio.errormsg(sender, is_frequency_valid.message)
+      return
+    end
+
     local meta = minetest.get_meta(pos)
     meta:set_string("frequency", fields.frequency)
     meta:set_string("rds_message", fields.rds_message)