diff --git a/pdf.js b/pdf.js
index 599e50af8..0a1fae18b 100644
--- a/pdf.js
+++ b/pdf.js
@@ -4357,7 +4357,13 @@ var PartialEvaluator = (function() {
       var lastChar = properties.lastChar;
       var glyphs = {};
       for (var i = firstChar; i <= lastChar; i++) {
-        var glyph = differences[i] || baseEncoding[i];
+        var glyph = differences[i];
+        if (!glyph) {
+          glyph = baseEncoding[i];
+          // skipping already specified by difference glyphs
+          if (differences.indexOf(glyph) >= 0)
+            continue;
+        }
         var index = GlyphsUnicode[glyph] || i;
         var width = properties.widths[i] || properties.widths[glyph];
         map[i] = {