diff --git a/doomsday/libs/core/include/de/data/string.h b/doomsday/libs/core/include/de/data/string.h
index 7d54602edd..5fdf8b5b79 100644
--- a/doomsday/libs/core/include/de/data/string.h
+++ b/doomsday/libs/core/include/de/data/string.h
@@ -58,21 +58,21 @@ struct DE_PUBLIC Sensitivity
 };
 
 enum PositionType { ByteOffset, CharacterOffset };
-    
+
 template <PositionType PosType>
 struct StronglyTypedPosition
 {
     dsize index;
 
     using Pos = StronglyTypedPosition<PosType>;
-    
+
     static constexpr PositionType type = PosType;
     static constexpr dsize        npos = std::numeric_limits<dsize>::max();
 
     explicit constexpr StronglyTypedPosition(dsize i = npos)
         : index(i)
     {}
-    
+
     explicit operator bool() const { return index != npos; }
 
     inline bool operator==(dsize i) const   { return index == i; }
@@ -101,14 +101,14 @@ struct StronglyTypedPosition
     Pos &operator++() { if (index != npos) index++; return *this; }
     Pos &operator--() { if (index != npos) index--; return *this; }
 };
-    
+
 using BytePos = StronglyTypedPosition<ByteOffset>;
 
 /**
  * Character index. A single character may be composed of multiple bytes.
  */
 using CharPos = StronglyTypedPosition<CharacterOffset>;
-    
+
 /**
  * Multibyte character iterator.
  */
@@ -144,7 +144,7 @@ struct DE_PUBLIC mb_iterator
     BytePos pos(const char *reference) const { return BytePos(i - reference); }
     BytePos pos(const String &reference) const;
 
-    Char decode() const;
+    Char decode(bool *ok = nullptr) const;
 };
 
 /**
diff --git a/doomsday/libs/core/src/data/string.cpp b/doomsday/libs/core/src/data/string.cpp
index e76b011477..d91e97dfc8 100644
--- a/doomsday/libs/core/src/data/string.cpp
+++ b/doomsday/libs/core/src/data/string.cpp
@@ -1102,7 +1102,7 @@ String String::fromPercentEncoding(const Block &percentEncoded) // static
 }
 
 //------------------------------------------------------------------------------------------------
-    
+
 String::const_reverse_iterator::const_reverse_iterator(const Range<const char *> &range)
     : iter(range.end, range.start)
 {
@@ -1133,7 +1133,7 @@ Char mb_iterator::operator*() const
     return decode();
 }
 
-Char mb_iterator::decode() const
+Char mb_iterator::decode(bool *ok) const
 {
     wchar_t ch = 0;
     curCharLen = std::mbrtowc(&ch, i, MB_CUR_MAX, &mb);
@@ -1143,6 +1143,11 @@ Char mb_iterator::decode() const
         // doesn't hang due to never reaching the end of the string.
         ch = '?';
         curCharLen = 1;
+        if (ok) *ok = false;
+    }
+    else
+    {
+        if (ok) *ok = true;
     }
     return ch;
 }
@@ -1173,7 +1178,9 @@ mb_iterator &mb_iterator::operator--()
     for (int j = 1; j < MB_CUR_MAX; ++j)
     {
         mb_iterator prec = i - j;
-        if (prec.decode() != 0 && prec.curCharLen == j)
+        bool ok;
+        prec.decode(&ok);
+        if (ok) // && prec.curCharLen == j)
         {
             prec.start = start;
             *this = prec;