diff --git a/contrib/harfbuzz-unicode.c b/contrib/harfbuzz-unicode.c
index 51dd4ea..cb7a85b 100644
--- a/contrib/harfbuzz-unicode.c
+++ b/contrib/harfbuzz-unicode.c
@@ -171,7 +171,10 @@ hb_utf16_script_run_prev(unsigned *num_code_points, HB_ScriptItem *output,
         current_script = script;
         continue;
       } else if (script == HB_Script_Inherited) {
-        current_script = script;
+        // Just assume that whatever follows this combining character is within
+        // the same script.  This is incorrect if you had language1 + combining
+        // char + language 2, but that is rare and this code is suspicious
+        // anyway.
         continue;
       } else {
         *iter = prev_iter;
diff --git a/src/harfbuzz-shaper.cpp b/src/harfbuzz-shaper.cpp
index f3ec8e1..2b0dfde 100644
--- a/src/harfbuzz-shaper.cpp
+++ b/src/harfbuzz-shaper.cpp
@@ -433,7 +433,7 @@ void HB_HeuristicSetGlyphAttributes(HB_ShaperItem *item)
 
     // ### zeroWidth and justification are missing here!!!!!
 
-    assert(item->num_glyphs <= length);
+    assert(length <= item->num_glyphs);
 
 //     qDebug("QScriptEngine::heuristicSetGlyphAttributes, num_glyphs=%d", item->num_glyphs);
     HB_GlyphAttributes *attributes = item->attributes;
@@ -451,7 +451,6 @@ void HB_HeuristicSetGlyphAttributes(HB_ShaperItem *item)
         }
         ++glyph_pos;
     }
-    assert(glyph_pos == item->num_glyphs);
 
     // first char in a run is never (treated as) a mark
     int cStart = 0;
diff --git a/src/harfbuzz-gpos.c b/src/harfbuzz-gpos.c
index 356dc01..db5ea0a 100644
--- a/src/harfbuzz-gpos.c
+++ b/src/harfbuzz-gpos.c
@@ -2976,6 +2976,9 @@ static HB_Error  Lookup_MarkMarkPos( GPOS_Instance*    gpi,
     j--;
   }
 
+  if ( i > buffer->in_pos )
+    return HB_Err_Not_Covered;
+
   error = _HB_OPEN_Coverage_Index( &mmp->Mark2Coverage, IN_GLYPH( j ),
                          &mark2_index );
   if ( error )