diff --git a/src/glyphset.mjs b/src/glyphset.mjs
index 3e5dab58..1d7ac43c 100644
--- a/src/glyphset.mjs
+++ b/src/glyphset.mjs
@@ -66,7 +66,7 @@ if(typeof Symbol !== 'undefined' && Symbol.iterator) {
  */
 GlyphSet.prototype.get = function(index) {
     // this.glyphs[index] is 'undefined' when low memory mode is on. glyph is pushed on request only.
-    if (this.glyphs[index] === undefined) {
+    if (this.glyphs[index] === undefined && typeof index === 'number') {
         this.font._push(index);
         if (typeof this.glyphs[index] === 'function') {
             this.glyphs[index] = this.glyphs[index]();
diff --git a/test/opentype.spec.mjs b/test/opentype.spec.mjs
index 26c2b211..e75dd6e8 100644
--- a/test/opentype.spec.mjs
+++ b/test/opentype.spec.mjs
@@ -174,6 +174,23 @@ describe('opentype.js on low memory mode', function() {
         assert.equal(ndGlyph.unicode, undefined);
     });
 
+    it('should return .notdef when looking up non-existent glyph in user-created font', function() {
+        const nullGlyph = new Glyph({
+            name: '.notdef',
+            path: new Path()
+        });
+        const font = new Font({
+            familyName: 'TestFont',
+            styleName: 'Medium',
+            unitsPerEm: 1000,
+            ascender: 800,
+            descender: -200,
+            glyphs: [nullGlyph]
+        });
+        const ndGlyph = font.charToGlyph('B');
+        assert.equal(ndGlyph.name, '.notdef');
+    });
+
     it('should correctly set unicode 0 for .null glyph', function() {
         const nullGlyph = new Glyph({
             name: '.null',