diff --git a/analyze_structure.lua b/analyze_structure.lua
index df9113e..62c7e81 100644
--- a/analyze_structure.lua
+++ b/analyze_structure.lua
@@ -48,13 +48,18 @@ local MINIMUM_NUMBER_OF_SET_ELEMENTS = 2
 
 --------------------------------------------------------------------------------
 
-local function is_identifier(str)
-    -- An identier is defined in the lua reference guide
+local function is_identifier(s)
+    -- Predicate: Is the given string usable as an identifier in this version of
+    -- Lua?
 
-    return str:match('^[_%a][_%w]*$') and not RESERVED_LUA_WORDS[str]
+    assert(type(s) == 'string')
+    --
+    return not not loadstring(s..'=0') and not s:find '%.' and not RESERVED_LUA_WORDS[s]
 end
 
 local function get_key_types (t)
+    assert(type(t) == 'table')
+    --
 	local types = { nr_types = -1 }
 	for key, _ in pairs(t) do
 		types[type(key)] = true
@@ -67,6 +72,8 @@ local function get_key_types (t)
 end
 
 local function get_value_types (t)
+    assert(type(t) == 'table')
+    --
     local types = { nr_types = -1 }
 	for _, value in pairs(t) do
 		types[type(value)] = true
@@ -81,6 +88,8 @@ end
 local function largest_number_index (t)
 	-- Returns the largest number index in t.
 
+    assert(type(t) == 'table')
+    --
 	local max_index = 0
 	for k,v in pairs(t) do
 		if type(k) == 'number' then
@@ -91,6 +100,8 @@ local function largest_number_index (t)
 end
 
 local function nr_elements_in_table (t)
+    assert(type(t) == 'table')
+    --
     local k, count = nil, -1
     repeat
         k, count = next(t, k), count + 1
diff --git a/test/test_pretty.lua b/test/test_pretty.lua
index 2d3acb8..6753ab1 100644
--- a/test/test_pretty.lua
+++ b/test/test_pretty.lua
@@ -96,7 +96,6 @@ format_test {
     input  = '\000',
     expect = '\'\\000\'',
 }
-
 format_test {
     input  = '\a\b\v\r\f',
     expect = '\'\\a\\b\\v\\r\\f\'',
@@ -107,6 +106,13 @@ format_test {
     expect = '\'ø\'',
 }
 
+format_test {
+    name   = 'Malformed Unicode is escaped',
+    input  = '\000\001\003\012\169\003\000\030',
+    expect = '\'\\000\\000\\001\\003\\012\\169\\003\\000\\030\'',
+}
+
+
 --------------------------------------------------------------------------------
 -- Primitive types
 
@@ -414,6 +420,12 @@ SUITE:addTest('UseCase: Can load function from file that is shortly deleted', fu
     assert(true)
 end)
 
+SUITE:addTest('UseCase: Can use pretty on loadstring block', function ()
+    -- TODO: Move to more appropriate test module.
+    format(loadstring 'hi = 0')
+    assert(true)
+end)
+
 --------------------------------------------------------------------------------
 
 return SUITE