diff --git a/contrib/harfbuzz-unicode.c b/contrib/harfbuzz-unicode.c
index 049e0ca..86458d0 100644
--- a/contrib/harfbuzz-unicode.c
+++ b/contrib/harfbuzz-unicode.c
@@ -120,7 +120,6 @@ hb_utf16_script_run_next(unsigned *num_code_points, HB_ScriptItem *output,
         current_script = script;
         continue;
       } else if (script == HB_Script_Inherited) {
-        current_script = script;
         continue;
       } else {
         *iter = prev_iter;
@@ -171,7 +170,10 @@ 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-arabic.c b/src/harfbuzz-arabic.c
index 3837087..ce2ca6c 100644
--- a/src/harfbuzz-arabic.c
+++ b/src/harfbuzz-arabic.c
@@ -1107,6 +1107,7 @@ HB_Bool HB_ArabicShape(HB_ShaperItem *item)
     assert(item->item.script == HB_Script_Arabic || item->item.script == HB_Script_Syriac
            || item->item.script == HB_Script_Nko);
 
+    item->shaperFlags |= HB_ShaperFlag_ForceMarksToZeroWidth;
 #ifndef NO_OPENTYPE
 
     if (HB_SelectScript(item, item->item.script == HB_Script_Arabic ? arabic_features : syriac_features)) {
diff --git a/src/harfbuzz-gpos.c b/src/harfbuzz-gpos.c
index a216005..7bd3b3b 100644
--- a/src/harfbuzz-gpos.c
+++ b/src/harfbuzz-gpos.c
@@ -3012,6 +3012,9 @@ 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 )
diff --git a/src/harfbuzz-shaper.cpp b/src/harfbuzz-shaper.cpp
index ce4d4ac..5999e08 100644
--- a/src/harfbuzz-shaper.cpp
+++ b/src/harfbuzz-shaper.cpp
@@ -430,8 +430,6 @@ void HB_HeuristicSetGlyphAttributes(HB_ShaperItem *item)
 
     // ### zeroWidth and justification are missing here!!!!!
 
-    assert(item->num_glyphs <= length);
-
 //     qDebug("QScriptEngine::heuristicSetGlyphAttributes, num_glyphs=%d", item->num_glyphs);
     HB_GlyphAttributes *attributes = item->attributes;
     unsigned short *logClusters = item->log_clusters;
@@ -448,7 +446,6 @@ 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;
@@ -1151,10 +1148,12 @@ HB_Bool HB_OpenTypeShape(HB_ShaperItem *item, const hb_uint32 *properties)
         return false;
     face->tmpLogClusters = tmpLogClusters;
 
+    const int itemLength = item->item.length;
+    assert(itemLength > 0);
     for (int i = 0; i < face->length; ++i) {
         hb_buffer_add_glyph(face->buffer, item->glyphs[i], properties ? properties[i] : 0, i);
         face->tmpAttributes[i] = item->attributes[i];
-        face->tmpLogClusters[i] = item->log_clusters[i];
+        face->tmpLogClusters[i] = i < itemLength ? item->log_clusters[i] : item->log_clusters[itemLength - 1];
     }
 
 #ifdef OT_DEBUG
@@ -1190,6 +1189,24 @@ HB_Bool HB_OpenTypeShape(HB_ShaperItem *item, const hb_uint32 *properties)
     return true;
 }
 
+/* See comments near the definition of HB_ShaperFlag_ForceMarksToZeroWidth for a description
+   of why this function exists. */
+void HB_FixupZeroWidth(HB_ShaperItem *item)
+{
+    HB_UShort property;
+
+    if (!item->face->gdef)
+        return;
+
+    for (unsigned int i = 0; i < item->num_glyphs; ++i) {
+        /* If the glyph is a mark, force its advance to zero. */
+        if (HB_GDEF_Get_Glyph_Property (item->face->gdef, item->glyphs[i], &property) == HB_Err_Ok &&
+            property == HB_GDEF_MARK) {
+            item->advances[i] = 0;
+        }
+    }
+}
+
 HB_Bool HB_OpenTypePosition(HB_ShaperItem *item, int availableGlyphs, HB_Bool doLogClusters)
 {
     HB_Face face = item->face;
@@ -1204,6 +1221,8 @@ HB_Bool HB_OpenTypePosition(HB_ShaperItem *item, int availableGlyphs, HB_Bool do
 
     if (!face->glyphs_substituted && !glyphs_positioned) {
         HB_GetGlyphAdvances(item);
+        if (item->face->current_flags & HB_ShaperFlag_ForceMarksToZeroWidth)
+            HB_FixupZeroWidth(item);
         return true; // nothing to do for us
     }
 
diff --git a/src/harfbuzz-shaper.h b/src/harfbuzz-shaper.h
index ab5c07a..72c9aa3 100644
--- a/src/harfbuzz-shaper.h
+++ b/src/harfbuzz-shaper.h
@@ -170,7 +170,11 @@ typedef enum {
 typedef enum {
     HB_ShaperFlag_Default = 0,
     HB_ShaperFlag_NoKerning = 1,
-    HB_ShaperFlag_UseDesignMetrics = 2
+    HB_ShaperFlag_UseDesignMetrics = 1 << 1,
+    /* Arabic vowels in some fonts (Times New Roman, at least) have
+       non-zero advances, when they should be zero.  Setting this shaper
+       flag causes us to zero out the advances for mark glyphs. */
+    HB_ShaperFlag_ForceMarksToZeroWidth = 1 << 2
 } HB_ShaperFlag;
 
 /*