diff --git a/source/utf8/checked.h b/source/utf8/checked.h
index 16bab4d..ecac556 100644
--- a/source/utf8/checked.h
+++ b/source/utf8/checked.h
@@ -204,10 +204,10 @@ namespace utf8
         while (start != end) {
             uint32_t cp = internal::mask16(*start++);
             // Take care of surrogate pairs first
-            if (internal::is_surrogate(cp)) {
+            if (internal::is_lead_surrogate(cp)) {
                 if (start != end) {
                     uint32_t trail_surrogate = internal::mask16(*start++);
-                    if (trail_surrogate >= internal::TRAIL_SURROGATE_MIN && trail_surrogate <= internal::TRAIL_SURROGATE_MAX)
+                    if (internal::is_trail_surrogate(trail_surrogate))
                         cp = (cp << 10) + trail_surrogate + internal::SURROGATE_OFFSET;
                     else
                         throw invalid_utf16(static_cast<uint16_t>(trail_surrogate));
@@ -216,6 +216,10 @@ namespace utf8
                     throw invalid_utf16(static_cast<uint16_t>(*start));
 
             }
+            // Lone trail surrogate
+            else if (internal::is_trail_surrogate(cp))
+                throw invalid_utf16(static_cast<uint16_t>(cp));
+
             result = append(cp, result);
         }
         return result;
diff --git a/source/utf8/core.h b/source/utf8/core.h
index 557ab0f..5a55f06 100644
--- a/source/utf8/core.h
+++ b/source/utf8/core.h
@@ -71,6 +71,18 @@ namespace internal
         return ((mask8(oc) >> 6) == 0x2);
     }
 
+    template <typename u16>
+    inline bool is_lead_surrogate(u16 cp)
+    {
+        return (cp >= LEAD_SURROGATE_MIN && cp <= LEAD_SURROGATE_MAX);
+    }
+
+    template <typename u16>
+    inline bool is_trail_surrogate(u16 cp)
+    {
+        return (cp >= TRAIL_SURROGATE_MIN && cp <= TRAIL_SURROGATE_MAX);
+    }
+
     template <typename u16>
     inline bool is_surrogate(u16 cp)
     {
diff --git a/source/utf8/unchecked.h b/source/utf8/unchecked.h
index 84207a5..d3110cb 100644
--- a/source/utf8/unchecked.h
+++ b/source/utf8/unchecked.h
@@ -132,7 +132,7 @@ namespace utf8
             while (start != end) {
                 uint32_t cp = internal::mask16(*start++);
             // Take care of surrogate pairs first
-                if (internal::is_surrogate(cp)) {
+                if (internal::is_lead_surrogate(cp)) {
                     uint32_t trail_surrogate = internal::mask16(*start++);
                     cp = (cp << 10) + trail_surrogate + internal::SURROGATE_OFFSET;
                 }