diff --git a/src/parser/parse.ts b/src/parser/parse.ts
index c58df745..eff97fb0 100644
--- a/src/parser/parse.ts
+++ b/src/parser/parse.ts
@@ -207,7 +207,7 @@ export class Parser<T extends ParserInput, TFlags extends OutputFlags<T['flags']
     for (const k of Object.keys(this.input.flags)) {
       const flag = this.input.flags[k]
       if (flags[k]) continue
-      if (flag.env) {
+      if (flag.env && Object.prototype.hasOwnProperty.call(process.env, flag.env)) {
         const input = process.env[flag.env]
         if (flag.type === 'option') {
           if (input) {
diff --git a/test/parser/parse.test.ts b/test/parser/parse.test.ts
index 1cd60587..54599390 100644
--- a/test/parser/parse.test.ts
+++ b/test/parser/parse.test.ts
@@ -910,6 +910,26 @@ See more help with --help`)
           delete process.env.TEST_FOO
         })
       }
+
+      it('ignores unset environment variables', async () => {
+        delete process.env.TEST_FOO
+        const out = await parse([], {
+          flags: {
+            foo: flags.boolean({env: 'TEST_FOO'}),
+          },
+        })
+        expect(out.flags.foo).to.be.undefined
+      })
+
+      it('uses default when environment variable is unset', async () => {
+        delete process.env.TEST_FOO
+        const out = await parse([], {
+          flags: {
+            foo: flags.boolean({env: 'TEST_FOO', default: true}),
+          },
+        })
+        expect(out.flags.foo).to.be.true
+      })
     })
   })