diff --git a/src/core/fonts.js b/src/core/fonts.js
index 5825a1470..993d3340b 100644
--- a/src/core/fonts.js
+++ b/src/core/fonts.js
@@ -484,8 +484,7 @@ function adjustMapping(charCodeToGlyphId, hasGlyph, newGlyphZeroId, toUnicode) {
   const isInPrivateArea = code =>
     (PRIVATE_USE_AREAS[0][0] <= code && code <= PRIVATE_USE_AREAS[0][1]) ||
     (PRIVATE_USE_AREAS[1][0] <= code && code <= PRIVATE_USE_AREAS[1][1]);
-  for (let originalCharCode in charCodeToGlyphId) {
-    originalCharCode |= 0;
+  for (const originalCharCode in charCodeToGlyphId) {
     let glyphId = charCodeToGlyphId[originalCharCode];
     // For missing glyphs don't create the mappings so the glyph isn't
     // drawn.
@@ -2834,7 +2833,7 @@ class Font {
       data: createPostTable(properties),
     };
 
-    const charCodeToGlyphId = [];
+    const charCodeToGlyphId = Object.create(null);
 
     // Helper function to try to skip mapping of empty glyphs.
     function hasGlyph(glyphId) {
diff --git a/test/pdfs/.gitignore b/test/pdfs/.gitignore
index e458cc908..733a26e08 100644
--- a/test/pdfs/.gitignore
+++ b/test/pdfs/.gitignore
@@ -242,6 +242,7 @@
 !ContentStreamNoCycleType3insideType3.pdf
 !ContentStreamCycleType3insideType3.pdf
 !issue2074.pdf
+!issue18117.pdf
 !scan-bad.pdf
 !issue13561_reduced.pdf
 !bug847420.pdf
diff --git a/test/pdfs/issue18117.pdf b/test/pdfs/issue18117.pdf
new file mode 100644
index 000000000..2dd6038b9
Binary files /dev/null and b/test/pdfs/issue18117.pdf differ
diff --git a/test/test_manifest.json b/test/test_manifest.json
index 3b959e751..74bc215d9 100644
--- a/test/test_manifest.json
+++ b/test/test_manifest.json
@@ -5406,6 +5406,20 @@
     "lastPage": 2,
     "type": "eq"
   },
+  {
+    "id": "issue18117",
+    "file": "pdfs/issue18117.pdf",
+    "md5": "0be6391f9910d807a0ded5e6913ff7e3",
+    "rounds": 1,
+    "type": "eq"
+  },
+  {
+    "id": "issue18117-text",
+    "file": "pdfs/issue18117.pdf",
+    "md5": "0be6391f9910d807a0ded5e6913ff7e3",
+    "rounds": 1,
+    "type": "text"
+  },
   {
     "id": "issue4890",
     "file": "pdfs/issue4890.pdf",