diff --git a/lslopt/lslbasefuncs.py b/lslopt/lslbasefuncs.py
index f32f3fa..8519287 100644
--- a/lslopt/lslbasefuncs.py
+++ b/lslopt/lslbasefuncs.py
@@ -475,6 +475,8 @@ def InternalUTF8toString(s):
                 # NOTE: Without the above line, the following one hits a bug in
                 # python-coverage. It IS executed but not detected.
                 continue
+            if LSO:
+                raise ELSONotSupported("Byte strings not supported")
             ret += u'?' * len(partialchar)
             partialchar = b''
             # fall through to process current character
@@ -482,11 +484,15 @@ def InternalUTF8toString(s):
             partialchar = c
             pending = 1 if o < 0xE0 else 2 if o < 0xF0 else 3
         elif o >= 0x80:
+            if LSO:
+                raise ELSONotSupported("Byte strings not supported")
             ret += u'?'
         else:
             ret += c.decode('utf8')
 
     if partialchar:
+        if LSO:
+            raise ELSONotSupported("Byte strings not supported")
         ret += u'?' * len(partialchar)
 
     return zstr(ret)
diff --git a/lslopt/lslcommon.py b/lslopt/lslcommon.py
index a4e351f..5bd285b 100644
--- a/lslopt/lslcommon.py
+++ b/lslopt/lslcommon.py
@@ -30,6 +30,9 @@ class Quaternion(tuple):
     def __repr__(self):
         return 'Quaternion(' + super(Quaternion, self).__repr__() + ')'
 
+class ELSONotSupported(Exception):
+    pass
+
 # Recognized: 3763, 6466, 6495
 Bugs = set([6495])