diff --git a/src/GlyphCache.cpp b/src/GlyphCache.cpp
index 282bdc18..14e37748 100644
--- a/src/GlyphCache.cpp
+++ b/src/GlyphCache.cpp
@@ -32,6 +32,7 @@ of the License or (at your option) any later version.
 #include "inc/GlyphFace.h"
 #include "inc/Endian.h"
 #include "inc/bits.h"
+#include <cassert>
 
 using namespace graphite2;
 
@@ -357,8 +358,9 @@ const GlyphFace * GlyphCache::Loader::read_glyph(unsigned short glyphid, GlyphFa
         if (_glyf)
         {
             int xMin, yMin, xMax, yMax;
-            size_t locidx = TtfUtil::LocaLookup(glyphid, _loca, _loca.size(), _head);
-            void *pGlyph = TtfUtil::GlyfLookup(_glyf, locidx, _glyf.size());
+            int locidx = TtfUtil::LocaLookup(glyphid, _loca, _loca.size(), _head);
+            assert(locidx >= 0);
+            void *pGlyph = TtfUtil::GlyfLookup(_glyf, static_cast<size_t>(locidx), _glyf.size());
 
             if (pGlyph && TtfUtil::GlyfBox(pGlyph, xMin, yMin, xMax, yMax))
             {
diff --git a/src/Silf.cpp b/src/Silf.cpp
index 44d3c961..ca2b9c9a 100644
--- a/src/Silf.cpp
+++ b/src/Silf.cpp
@@ -309,7 +309,7 @@ uint16 Silf::findPseudo(uint32 uid) const
     return 0;
 }
 
-uint16 Silf::findClassIndex(uint16 cid, uint16 gid) const
+int Silf::findClassIndex(uint16 cid, uint16 gid) const
 {
     if (cid > m_nClass) return -1;
 
diff --git a/src/TtfUtil.cpp b/src/TtfUtil.cpp
index 2eb46a11..9c00e732 100644
--- a/src/TtfUtil.cpp
+++ b/src/TtfUtil.cpp
@@ -1211,7 +1211,7 @@ unsigned int CmapSubtable12NextCodepoint(const void *pCmap310, unsigned int nUni
     Technically this method should return an unsigned long but it is unlikely the offset will
         exceed 2^31.
 ----------------------------------------------------------------------------------------------*/
-size_t LocaLookup(gid16 nGlyphId,
+int LocaLookup(gid16 nGlyphId,
         const void * pLoca, size_t lLocaSize,
         const void * pHead) // throw (std::out_of_range)
 {
diff --git a/src/inc/Segment.h b/src/inc/Segment.h
index 6cf83408..fd45efb3 100644
--- a/src/inc/Segment.h
+++ b/src/inc/Segment.h
@@ -115,7 +115,7 @@ class Segment
     void associateChars(int offset, size_t num);
     void linkClusters(Slot *first, Slot *last);
     uint16 getClassGlyph(uint16 cid, uint16 offset) const { return m_silf->getClassGlyph(cid, offset); }
-    uint16 findClassIndex(uint16 cid, uint16 gid) const { return m_silf->findClassIndex(cid, gid); }
+    int findClassIndex(uint16 cid, uint16 gid) const { return m_silf->findClassIndex(cid, gid); }
     int addFeatures(const Features& feats) { m_feats.push_back(feats); return int(m_feats.size()) - 1; }
     uint32 getFeature(int index, uint8 findex) const { const FeatureRef* pFR=m_face->theSill().theFeatureMap().featureRef(findex); if (!pFR) return 0; else return pFR->getFeatureVal(m_feats[index]); }
     void setFeature(int index, uint8 findex, uint32 val) {
diff --git a/src/inc/Silf.h b/src/inc/Silf.h
index edc0c3a1..5df6b4ed 100644
--- a/src/inc/Silf.h
+++ b/src/inc/Silf.h
@@ -76,7 +76,7 @@ class Silf
 
     bool readGraphite(const byte * const pSilf, size_t lSilf, Face &face, uint32 version);
     bool runGraphite(Segment *seg, uint8 firstPass=0, uint8 lastPass=0, int dobidi = 0) const;
-    uint16 findClassIndex(uint16 cid, uint16 gid) const;
+    int findClassIndex(uint16 cid, uint16 gid) const;
     uint16 getClassGlyph(uint16 cid, unsigned int index) const;
     uint16 findPseudo(uint32 uid) const;
     uint8 numUser() const { return m_aUser; }
diff --git a/src/inc/TtfUtil.h b/src/inc/TtfUtil.h
index 3952bc06..4c109c0e 100644
--- a/src/inc/TtfUtil.h
+++ b/src/inc/TtfUtil.h
@@ -153,7 +153,7 @@ class Tag
         const void * pHhea, int & nLsb, unsigned int & nAdvWid);
 
     ////////////////////////////////// primitives for loca and glyf lookup
-    size_t LocaLookup(gid16 nGlyphId, const void * pLoca, size_t lLocaSize,
+    int LocaLookup(gid16 nGlyphId, const void * pLoca, size_t lLocaSize,
         const void * pHead); // throw (std::out_of_range);
     void * GlyfLookup(const void * pGlyf, size_t lGlyfOffset, size_t lTableLen);
 
diff --git a/src/inc/bits.h b/src/inc/bits.h
index 9365986a..dde2cae4 100644
--- a/src/inc/bits.h
+++ b/src/inc/bits.h
@@ -79,7 +79,7 @@ inline unsigned int bit_set_count(signed long long v)
 template<typename T>
 inline unsigned int bit_set_count(T v)
 {
-	static size_t const ONES = ~0;
+	static size_t const ONES = static_cast<size_t>(~0);
 
 	v = v - ((v >> 1) & T(ONES/3));                      // temp
     v = (v & T(ONES/15*3)) + ((v >> 2) & T(ONES/15*3));  // temp
diff --git a/src/inc/opcodes.h b/src/inc/opcodes.h
index ff2f1741..3e76ffcb 100644
--- a/src/inc/opcodes.h
+++ b/src/inc/opcodes.h
@@ -24,6 +24,9 @@ Mozilla Public License (http://mozilla.org/MPL) or the GNU General Public
 License, as published by the Free Software Foundation, either version 2
 of the License or (at your option) any later version.
 */
+
+#include <cassert>
+
 #pragma once
 // This file will be pulled into and integrated into a machine implmentation
 // DO NOT build directly and under no circumstances ever #include headers in
@@ -233,12 +236,13 @@ STARTOP(put_subs_8bit_obs)
     const int           slot_ref     = int8(param[0]);
     const unsigned int  input_class  = uint8(param[1]),
                         output_class = uint8(param[2]);
-    uint16 index;
+    int index;
     slotref slot = slotat(slot_ref);
     if (slot)
     {
         index = seg.findClassIndex(input_class, slot->gid());
-        is->setGlyph(&seg, seg.getClassGlyph(output_class, index));
+        asset(index >= 0);
+        is->setGlyph(&seg, seg.getClassGlyph(output_class, static_cast<uint16>(index)));
     }
 ENDOP
 
@@ -601,7 +605,8 @@ STARTOP(put_subs)
     if (slot)
     {
         int index = seg.findClassIndex(input_class, slot->gid());
-        is->setGlyph(&seg, seg.getClassGlyph(output_class, index));
+        assert(index >=0 );
+        is->setGlyph(&seg, seg.getClassGlyph(output_class, static_cast<uint16>(index)));
     }
 ENDOP