diff --git a/src/cmap/commands.ts b/src/cmap/commands.ts
index 78e4445f25..a49d184319 100644
--- a/src/cmap/commands.ts
+++ b/src/cmap/commands.ts
@@ -484,15 +484,15 @@ export class Response {
   responseTo: number;
   opCode: number;
   fromCompressed?: boolean;
-  responseFlags: number;
-  cursorId: Long;
-  startingFrom: number;
-  numberReturned: number;
-  documents: (Document | Buffer)[];
-  cursorNotFound: boolean;
-  queryFailure: boolean;
-  shardConfigStale: boolean;
-  awaitCapable: boolean;
+  responseFlags?: number;
+  cursorId?: Long;
+  startingFrom?: number;
+  numberReturned?: number;
+  documents: (Document | Buffer)[] = new Array(0);
+  cursorNotFound?: boolean;
+  queryFailure?: boolean;
+  shardConfigStale?: boolean;
+  awaitCapable?: boolean;
   promoteLongs: boolean;
   promoteValues: boolean;
   promoteBuffers: boolean;
@@ -522,20 +522,7 @@ export class Response {
     this.opCode = msgHeader.opCode;
     this.fromCompressed = msgHeader.fromCompressed;
 
-    // Read the message body
-    this.responseFlags = msgBody.readInt32LE(0);
-    this.cursorId = new BSON.Long(msgBody.readInt32LE(4), msgBody.readInt32LE(8));
-    this.startingFrom = msgBody.readInt32LE(12);
-    this.numberReturned = msgBody.readInt32LE(16);
-
-    // Preallocate document array
-    this.documents = new Array(this.numberReturned);
-
     // Flag values
-    this.cursorNotFound = (this.responseFlags & CURSOR_NOT_FOUND) !== 0;
-    this.queryFailure = (this.responseFlags & QUERY_FAILURE) !== 0;
-    this.shardConfigStale = (this.responseFlags & SHARD_CONFIG_STALE) !== 0;
-    this.awaitCapable = (this.responseFlags & AWAIT_CAPABLE) !== 0;
     this.promoteLongs = typeof this.opts.promoteLongs === 'boolean' ? this.opts.promoteLongs : true;
     this.promoteValues =
       typeof this.opts.promoteValues === 'boolean' ? this.opts.promoteValues : true;
@@ -574,6 +561,20 @@ export class Response {
     // (See https://docs.mongodb.com/manual/reference/mongodb-wire-protocol/#wire-op-reply)
     this.index = 20;
 
+    // Read the message body
+    this.responseFlags = this.data.readInt32LE(0);
+    this.cursorId = new BSON.Long(this.data.readInt32LE(4), this.data.readInt32LE(8));
+    this.startingFrom = this.data.readInt32LE(12);
+    this.numberReturned = this.data.readInt32LE(16);
+
+    // Preallocate document array
+    this.documents = new Array(this.numberReturned);
+
+    this.cursorNotFound = (this.responseFlags & CURSOR_NOT_FOUND) !== 0;
+    this.queryFailure = (this.responseFlags & QUERY_FAILURE) !== 0;
+    this.shardConfigStale = (this.responseFlags & SHARD_CONFIG_STALE) !== 0;
+    this.awaitCapable = (this.responseFlags & AWAIT_CAPABLE) !== 0;
+
     // Parse Body
     for (let i = 0; i < this.numberReturned; i++) {
       bsonSize =
diff --git a/test/unit/cmap/commands.test.js b/test/unit/cmap/commands.test.js
new file mode 100644
index 0000000000..65993f3c33
--- /dev/null
+++ b/test/unit/cmap/commands.test.js
@@ -0,0 +1,111 @@
+const { expect } = require('chai');
+const { Response } = require('../../../src/cmap/commands');
+
+describe('commands', function () {
+  describe('Response', function () {
+    describe('#parse', function () {
+      context('when the message body is invalid', function () {
+        context('when the buffer is empty', function () {
+          const message = Buffer.from([]);
+          const header = {
+            length: 0,
+            requestId: 0,
+            responseTo: 0,
+            opCode: 0
+          };
+          const body = Buffer.from([]);
+
+          it('throws an exception', function () {
+            const response = new Response(message, header, body);
+            expect(() => response.parse()).to.throw(RangeError, /outside buffer bounds/);
+          });
+        });
+
+        context('when numReturned is invalid', function () {
+          const message = Buffer.from([]);
+          const header = {
+            length: 0,
+            requestId: 0,
+            responseTo: 0,
+            opCode: 0
+          };
+          const body = Buffer.alloc(5 * 4);
+          body.writeInt32LE(-1, 16);
+
+          it('throws an exception', function () {
+            const response = new Response(message, header, body);
+            expect(() => response.parse()).to.throw(RangeError, /Invalid array length/);
+          });
+        });
+      });
+    });
+
+    describe('#constructor', function () {
+      context('when the message body is invalid', function () {
+        const message = Buffer.from([]);
+        const header = {
+          length: 0,
+          requestId: 0,
+          responseTo: 0,
+          opCode: 0
+        };
+        const body = Buffer.from([]);
+
+        it('does not throw an exception', function () {
+          let error;
+          try {
+            new Response(message, header, body);
+          } catch (err) {
+            error = err;
+          }
+          expect(error).to.be.undefined;
+        });
+
+        it('initializes the documents to an empty array', function () {
+          const response = new Response(message, header, body);
+          expect(response.documents).to.be.empty;
+        });
+
+        it('does not set the responseFlags', function () {
+          const response = new Response(message, header, body);
+          expect(response.responseFlags).to.be.undefined;
+        });
+
+        it('does not set the cursorNotFound flag', function () {
+          const response = new Response(message, header, body);
+          expect(response.cursorNotFound).to.be.undefined;
+        });
+
+        it('does not set the cursorId', function () {
+          const response = new Response(message, header, body);
+          expect(response.cursorId).to.be.undefined;
+        });
+
+        it('does not set startingFrom', function () {
+          const response = new Response(message, header, body);
+          expect(response.startingFrom).to.be.undefined;
+        });
+
+        it('does not set numberReturned', function () {
+          const response = new Response(message, header, body);
+          expect(response.numberReturned).to.be.undefined;
+        });
+
+        it('does not set queryFailure', function () {
+          const response = new Response(message, header, body);
+          expect(response.queryFailure).to.be.undefined;
+        });
+
+        it('does not set shardConfigStale', function () {
+          const response = new Response(message, header, body);
+          expect(response.shardConfigStale).to.be.undefined;
+        });
+
+        it('does not set awaitCapable', function () {
+          const response = new Response(message, header, body);
+          expect(response.awaitCapable).to.be.undefined;
+        });
+      });
+    });
+  });
+});