diff --git a/booster/booster/locale/utf.h b/booster/booster/locale/utf.h
index 069cd19..c7dc024 100644
--- a/booster/booster/locale/utf.h
+++ b/booster/booster/locale/utf.h
@@ -219,16 +219,22 @@ namespace utf {
                 if(BOOSTER_LOCALE_UNLIKELY(p==e))
                     return incomplete;
                 tmp = *p++;
+                if (!is_trail(tmp))
+                    return illegal;
                 c = (c << 6) | ( tmp & 0x3F);
             case 2:
                 if(BOOSTER_LOCALE_UNLIKELY(p==e))
                     return incomplete;
                 tmp = *p++;
+                if (!is_trail(tmp))
+                    return illegal;
                 c = (c << 6) | ( tmp & 0x3F);
             case 1:
                 if(BOOSTER_LOCALE_UNLIKELY(p==e))
                     return incomplete;
                 tmp = *p++;
+                if (!is_trail(tmp))
+                    return illegal;
                 c = (c << 6) | ( tmp & 0x3F);
             }
 
diff --git a/booster/lib/locale/test/test_codepage_converter.cpp b/booster/lib/locale/test/test_codepage_converter.cpp
index aabab73..d48b5bd 100644
--- a/booster/lib/locale/test/test_codepage_converter.cpp
+++ b/booster/lib/locale/test/test_codepage_converter.cpp
@@ -140,6 +140,20 @@ int main()
         TEST_TO("\xf8\x90\x80\x80\x80",illegal);  // 400 0000
         TEST_TO("\xfd\xbf\xbf\xbf\xbf\xbf",illegal);  // 7fff ffff
 
+        std::cout << "-- Invalid trail" << std::endl;
+        TEST_TO("\xC2\x7F",illegal);
+        TEST_TO("\xdf\x7F",illegal);
+        TEST_TO("\xe0\x7F\x80",illegal);
+        TEST_TO("\xef\xbf\x7F",illegal);
+        TEST_TO("\xe0\x7F\x80",illegal);
+        TEST_TO("\xef\xbf\x7F",illegal);
+        TEST_TO("\xf0\x7F\x80\x80",illegal);
+        TEST_TO("\xf4\x7f\xbf\xbf",illegal);
+        TEST_TO("\xf0\x90\x7F\x80",illegal);
+        TEST_TO("\xf4\x8f\x7F\xbf",illegal);
+        TEST_TO("\xf0\x90\x80\x7F",illegal);
+        TEST_TO("\xf4\x8f\xbf\x7F",illegal);
+
         std::cout << "-- Invalid length" << std::endl;
 
         /// Test that this actually works
diff --git a/private/utf_iterator.h b/private/utf_iterator.h
index f15c4eb..f8205b3 100644
--- a/private/utf_iterator.h
+++ b/private/utf_iterator.h
@@ -25,7 +25,12 @@ namespace utf {
 }
 
 namespace utf8 {
-        inline int trail_length(unsigned char c) 
+        inline bool is_trail(char ci)
+        {
+            unsigned char c=ci;
+            return (c & 0xC0)==0x80;
+        }
+	inline int trail_length(unsigned char c) 
         {
             if(c < 128)
                 return 0;
@@ -91,16 +96,22 @@ namespace utf8 {
 				if(p==e)
 					return illegal;
 				tmp = *p++;
+				if (!is_trail(tmp))
+					return illegal;
 				c = (c << 6) | ( tmp & 0x3F);
 			case 2:
 				if(p==e)
 					return illegal;
 				tmp = *p++;
+				if (!is_trail(tmp))
+					return illegal;
 				c = (c << 6) | ( tmp & 0x3F);
 			case 1:
 				if(p==e)
 					return illegal;
 				tmp = *p++;
+				if (!is_trail(tmp))
+					return illegal;
 				c = (c << 6) | ( tmp & 0x3F);
 		}
 
diff --git a/tests/form_test.py b/tests/form_test.py
index 2f83bf6..c85c524 100755
--- a/tests/form_test.py
+++ b/tests/form_test.py
@@ -127,6 +127,10 @@ test_valid('text5','/text','_1=%d7%a9%d6%b8%d7%9c%d7%95%d7%9d','valid\nשָלו
 test_valid('text6','/text','_1=%d7%a9%d7%9c','valid\nשל')
 test_valid('text7','/text','_1=%FF%FF','invalid\n\xFF\xFF')
 test_valid('text8','/text','_1=%01%01','invalid\n\x01\x01')
+test_valid('text9.1','/text','_1=xx%DF%7F','invalid\nxx\xDF\x7F')
+test_valid('text9.2','/text','_1=xx%C2%7F','invalid\nxx\xC2\x7F')
+test_valid('text9.3','/text','_1=xx%e0%7F%80','invalid\nxx\xe0\x7F\x80')
+test_valid('text9.4','/text','_1=xx%f0%7F%80%80','invalid\nxx\xf0\x7F\x80\x80')
 
 test_valid('number','/number','_1=','invalid\n')
 test_valid('number1','/number','_1=10','valid\n10')