diff --git a/lib/validateOptions.ts b/lib/validateOptions.ts
index 9bdb236..331dfb7 100644
--- a/lib/validateOptions.ts
+++ b/lib/validateOptions.ts
@@ -27,6 +27,10 @@ const validateOptions = function ({ options, optionDefinitions }: {
         break;
       }
       case 'number': {
+        if (!optionRequired && value === undefined) {
+          break;
+        }
+
         if (typeof value !== 'number' || Number.isNaN(value)) {
           throw new errors.OptionInvalid(`Option '${optionDefinition.name}' must be a number.`, {
             data: { optionDefinition }
diff --git a/test/unit/validateOptionsTests.ts b/test/unit/validateOptionsTests.ts
index 7cfc5d4..2194e55 100644
--- a/test/unit/validateOptionsTests.ts
+++ b/test/unit/validateOptionsTests.ts
@@ -1,6 +1,6 @@
 import { assert } from 'assertthat';
 import { CustomError } from 'defekt';
-import { OptionDefinition } from '../../lib/elements/OptionDefinition';
+import { OptionDefinition } from '../../lib';
 import { validateOptions } from 'lib/validateOptions';
 
 suite('validateOptions', (): void => {
@@ -78,6 +78,23 @@ suite('validateOptions', (): void => {
       }).is.not.throwing();
     });
 
+    test('accepts a missing, non-required number.', async (): Promise<void> => {
+      const options = {
+        option: undefined
+      };
+      const optionDefinitions: OptionDefinition[] = [
+        {
+          name: 'option',
+          type: 'number',
+          isRequired: false
+        }
+      ];
+
+      assert.that((): void => {
+        validateOptions({ options, optionDefinitions });
+      }).is.not.throwing();
+    });
+
     test('throws an exception if a number option is NaN.', async (): Promise<void> => {
       const options = {
         option: Number.NaN