diff --git a/assert-gooder.lua b/assert-gooder.lua
index d060d0e..8af2c3d 100644
--- a/assert-gooder.lua
+++ b/assert-gooder.lua
@@ -154,7 +154,7 @@ local function get_module_filetext (module_filepath)
     -- Just attempt standard file open
     local filehandle = io.open(module_filepath, 'r')
     if filehandle then
-        filetext = filehandle:read '*all'
+        local filetext = filehandle:read '*all'
         filehandle:close()
         return filetext
     end
@@ -167,6 +167,26 @@ local function get_module_filetext (module_filepath)
     return nil
 end
 
+local function seperate_by_toplevel_commas (text)
+    assert(type(text) == 'string')
+    local section_start, index, sections = 1, 1, {}
+    while index < #text do
+        local next_comma = text:find(',', index)
+        local next_par_start, next_par_end = text:find('%b()', index)
+        if not next_comma then
+            break
+        elseif not next_par_start or next_comma < next_par_start then
+            sections[#sections+1] = text:sub(section_start, next_comma - 1)
+            index = next_comma + 1
+            section_start = index
+        else
+            index = next_par_end + 1
+        end
+    end
+    sections[#sections+1] = text:sub(section_start)
+    return sections
+end
+
 local function get_assert_body_text (call_info)
     if call_info.what == 'Lua' or call_info.what == 'main' then
         -- Find filetext
@@ -190,7 +210,9 @@ local function get_assert_body_text (call_info)
             end
         end
         -- Find body exclusively.
-        return table.concat(lines_after, '\n'):match('assert%s*(%b())'):sub(2, -2)
+        local assert_arguments_text = table.concat(lines_after, '\n'):match('assert%s*(%b())'):sub(2, -2)
+        local assert_arguments = seperate_by_toplevel_commas(assert_arguments_text)
+        return assert_arguments[1]
     end
 
     error 'Not implemented yet!'
@@ -365,7 +387,7 @@ local function determine_error_message (call_info, msg, condition)
     end
 end
 
-return function (condition)
+return function (condition, format, ...)
     if condition then  return  condition  end
     --
     local level     = 2
@@ -383,15 +405,26 @@ return function (condition)
     if not success then
         io.stderr:write(('[assert-gooder/internal]: Internal error occured while determining error message for calling assert:\n    %s\n'):format(internal_error_msg))
     end
-    --
+
+    -- Format error message:
     assert(#msg_container <= 2 and type(msg_container[1]) == 'string')
-    local l = {'assertion failed! ', msg_container[1]}
+    local l = {}
+    if format ~= nil then
+        l[#l+1] = (type(format) == 'string') and format:format(...) or tostring(format)
+        l[#l+1] = ':'
+    else
+        l[#l+1] = 'assertion failed!'
+    end
+    l[#l+1] = ' '
+    l[#l+1] = msg_container[1]
     if msg_container[2] then
         assert(type(msg_container[2]) == 'string')
-        l[3] = ' ('
-        l[4] = msg_container[2]
-        l[5] = ')'
+        l[#l+1] = ' ('
+        l[#l+1] = msg_container[2]
+        l[#l+1] = ')'
     end
+
+    -- Throw error message
     error(table.concat(l, ''), 2)
 end
 
diff --git a/test/test_assert-gooder.lua b/test/test_assert-gooder.lua
index f20c62e..eb5ba4d 100644
--- a/test/test_assert-gooder.lua
+++ b/test/test_assert-gooder.lua
@@ -303,6 +303,27 @@ SUITE:addTest('Identify odd number', function ()
 	assert_equal('./test/test_assert-gooder.lua:'..curline(-2)..': '..'assertion failed! bad local \'a\' (odd number expected, but got even number 5.21)', msg)
 end)
 
+--------------------------------------------------------------------------------
+-- Custom error message
+
+SUITE:addTest('Custom error message', function ()
+	local _, msg  =  pcall(function ()
+		local a = 2
+		assert(type(a) == 'string', 'expected string')
+	end)
+	assert_equal('./test/test_assert-gooder.lua:'..curline(-2)..': '..'expected string: bad local \'a\' (string expected, but got number 2)', msg)
+end)
+
+SUITE:addTest('Custom formatted message', function ()
+	local _, msg  =  pcall(function ()
+		local a = 2
+		assert(type(a) == 'string', 'expected string not %s', type(a))
+	end)
+	assert_equal('./test/test_assert-gooder.lua:'..curline(-2)..': '..'expected string not number: bad local \'a\' (string expected, but got number 2)', msg)
+end)
+
+
+
 
 --------------------------------------------------------------------------------