diff --git a/src/core/fonts.js b/src/core/fonts.js
index 9db048959..c03fbb62f 100644
--- a/src/core/fonts.js
+++ b/src/core/fonts.js
@@ -2901,14 +2901,21 @@ var Font = (function FontClosure() {
         }
 
         // Last, try to map any missing charcodes using the post table.
-        if (properties.glyphNames && baseEncoding.length) {
+        if (
+          properties.glyphNames &&
+          (baseEncoding.length || this.differences.length)
+        ) {
           for (let i = 0; i < 256; ++i) {
-            if (charCodeToGlyphId[i] === undefined && baseEncoding[i]) {
-              glyphName = baseEncoding[i];
-              const glyphId = properties.glyphNames.indexOf(glyphName);
-              if (glyphId > 0 && hasGlyph(glyphId)) {
-                charCodeToGlyphId[i] = glyphId;
-              }
+            if (charCodeToGlyphId[i] !== undefined) {
+              continue;
+            }
+            glyphName = this.differences[i] || baseEncoding[i];
+            if (!glyphName) {
+              continue;
+            }
+            const glyphId = properties.glyphNames.indexOf(glyphName);
+            if (glyphId > 0 && hasGlyph(glyphId)) {
+              charCodeToGlyphId[i] = glyphId;
             }
           }
         }
diff --git a/test/pdfs/.gitignore b/test/pdfs/.gitignore
index 12f5eb3ff..15f105b38 100644
--- a/test/pdfs/.gitignore
+++ b/test/pdfs/.gitignore
@@ -158,6 +158,7 @@
 !issue6019.pdf
 !issue6621.pdf
 !issue6286.pdf
+!issue13107_reduced.pdf
 !issue1055r.pdf
 !issue11713.pdf
 !issue1293r.pdf
diff --git a/test/pdfs/issue13107_reduced.pdf b/test/pdfs/issue13107_reduced.pdf
new file mode 100644
index 000000000..60e59492f
Binary files /dev/null and b/test/pdfs/issue13107_reduced.pdf differ
diff --git a/test/test_manifest.json b/test/test_manifest.json
index 0800d4c37..319959420 100644
--- a/test/test_manifest.json
+++ b/test/test_manifest.json
@@ -595,6 +595,12 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue13107",
+      "file": "pdfs/issue13107_reduced.pdf",
+      "md5": "13c8d12223dec2752d3c90d250db32b7",
+      "rounds": 1,
+      "type": "eq"
+    },
     {  "id": "bug1186827",
        "file": "pdfs/bug1186827.pdf",
        "md5": "6c5526ae1a9d66cb517153001afc196e",