diff --git a/src/core/evaluator.js b/src/core/evaluator.js
index 1fff0754392dd..17a73f5690fdd 100644
--- a/src/core/evaluator.js
+++ b/src/core/evaluator.js
@@ -3017,7 +3017,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
           // is a tagged pdf. Create a barbebones one to get by.
           descriptor = new Dict(null);
           descriptor.set("FontName", Name.get(type));
-          descriptor.set("FontBBox", dict.getArray("FontBBox"));
+          descriptor.set("FontBBox", dict.getArray("FontBBox") || [0, 0, 0, 0]);
         } else {
           // Before PDF 1.5 if the font was one of the base 14 fonts, having a
           // FontDescriptor was not required.
diff --git a/src/core/primitives.js b/src/core/primitives.js
index 6ec9e4eecadbc..b3c5971249d5a 100644
--- a/src/core/primitives.js
+++ b/src/core/primitives.js
@@ -14,7 +14,7 @@
  */
 /* uses XRef */
 
-import { assert } from "../shared/util.js";
+import { assert, unreachable } from "../shared/util.js";
 
 var EOF = {};
 
@@ -85,9 +85,9 @@ var Dict = (function DictClosure() {
     // automatically dereferences Ref objects
     get(key1, key2, key3) {
       let value = this._map[key1];
-      if (value === undefined && !(key1 in this._map) && key2 !== undefined) {
+      if (value === undefined && key2 !== undefined) {
         value = this._map[key2];
-        if (value === undefined && !(key2 in this._map) && key3 !== undefined) {
+        if (value === undefined && key3 !== undefined) {
           value = this._map[key3];
         }
       }
@@ -100,9 +100,9 @@ var Dict = (function DictClosure() {
     // Same as get(), but returns a promise and uses fetchIfRefAsync().
     async getAsync(key1, key2, key3) {
       let value = this._map[key1];
-      if (value === undefined && !(key1 in this._map) && key2 !== undefined) {
+      if (value === undefined && key2 !== undefined) {
         value = this._map[key2];
-        if (value === undefined && !(key2 in this._map) && key3 !== undefined) {
+        if (value === undefined && key3 !== undefined) {
           value = this._map[key3];
         }
       }
@@ -138,11 +138,18 @@ var Dict = (function DictClosure() {
     },
 
     set: function Dict_set(key, value) {
+      if (
+        (typeof PDFJSDev === "undefined" ||
+          PDFJSDev.test("!PRODUCTION || TESTING")) &&
+        value === undefined
+      ) {
+        unreachable('Dict.set: The "value" cannot be undefined.');
+      }
       this._map[key] = value;
     },
 
     has: function Dict_has(key) {
-      return key in this._map;
+      return this._map[key] !== undefined;
     },
 
     forEach: function Dict_forEach(callback) {
diff --git a/test/unit/primitives_spec.js b/test/unit/primitives_spec.js
index bb54c2e7cc3f4..219a811294223 100644
--- a/test/unit/primitives_spec.js
+++ b/test/unit/primitives_spec.js
@@ -121,11 +121,13 @@ describe("primitives", function() {
       checkInvalidKeyValues(dictWithSizeKey);
     });
 
-    it("should return correct value for stored Size key with undefined value", function() {
-      var dict = new Dict();
-      dict.set("Size");
+    it("should not accept to set a key with an undefined value", function() {
+      const dict = new Dict();
+      expect(function() {
+        dict.set("Size");
+      }).toThrow(new Error('Dict.set: The "value" cannot be undefined.'));
 
-      expect(dict.has("Size")).toBeTruthy();
+      expect(dict.has("Size")).toBeFalsy();
 
       checkInvalidKeyValues(dict);
     });