diff --git a/__tests__/utils/isValidKey.test.ts b/__tests__/utils/isValidKey.test.ts
new file mode 100644
index 0000000..f51e242
--- /dev/null
+++ b/__tests__/utils/isValidKey.test.ts
@@ -0,0 +1,105 @@
+import { Chance } from 'chance';
+import { isValidKey } from '../../src/utils/isValidKey';
+import { firstOnlyCharacters, middleOnlyCharacters, reservedCharacters } from '../../src/constants/characterSets';
+
+describe('`isValidKey`', () => {
+  let chance: Chance.Chance;
+
+  beforeEach(() => {
+    chance = new Chance();
+  });
+
+  it('should throw errors that correctly reference the decorator name and key', () => {
+    const decoratorName = chance.string();
+    const key = chance.integer();
+
+    try {
+      isValidKey(decoratorName, key);
+    } catch (error) {
+      expect(error.message).toContain(
+        `@${decoratorName} can only be applied to specific keys. Expected key ${key.toString()}`,
+      );
+    }
+  });
+
+  it('should throw an error if the key is not a string', () => {
+    const key = chance.pickone([Symbol(), chance.integer()]);
+
+    try {
+      isValidKey('some-decorator', key);
+    } catch (error) {
+      expect(error.message).toContain(
+        `Expected key ${key.toString()} to be of type string, but received ${typeof key}.`,
+      );
+    }
+  });
+
+  it('should throw an error if the key is an empty string', () => {
+    const key = '';
+
+    try {
+      isValidKey('some-decorator', key);
+    } catch (error) {
+      expect(error.message).toContain('Expected key to be non-empty.');
+    }
+  });
+
+  it('should throw an error if the key contains reserved characters', () => {
+    const key = chance.pickone([...reservedCharacters]);
+
+    try {
+      isValidKey('some-decorator', key);
+    }
+    catch (error) {
+      expect(error.message).toContain(
+        `Expected key ${key} to not contain any reserved characters, but found ${key}.`,
+      );
+    }
+  });
+
+  it('should throw an error if the key contains first only characters after the first character', () => {
+    const firstOnlyCharacter = chance.pickone([...firstOnlyCharacters]);
+    const key = `${chance.guid()}${firstOnlyCharacter}`;
+
+    try {
+      isValidKey('some-decorator', key);
+    } catch (error) {
+      expect(error.message).toContain(
+        `Expected key ${key} to not contain the characters ${firstOnlyCharacter}, except as the first character.`,
+      );
+    }
+  });
+
+  it('should allow first only characters as the first character', () => {
+    const firstOnlyCharacter = chance.pickone([...firstOnlyCharacters]);
+    const key = `${firstOnlyCharacter}${chance.guid()}`;
+
+    expect(isValidKey('some-decorator', key)).toBe(true);
+  });
+
+  it('should throw an error if the key contains middle only characters as the first or last character', () => {
+    const middleOnlyCharacter = chance.pickone([...middleOnlyCharacters]);
+    const key = `${middleOnlyCharacter}${chance.guid()}${middleOnlyCharacter}`;
+
+    try {
+      isValidKey('some-decorator', key);
+    } catch (error) {
+      expect(error.message).toContain(
+        `Expected key ${key} to not contain the characters ${middleOnlyCharacter}, ${middleOnlyCharacter} as the first or last character.`,
+      );
+    }
+  });
+
+  it('should allow middle only characters in the middle of the key', () => {
+    const middleOnlyCharacter = chance.pickone([...middleOnlyCharacters]);
+    const key = `${chance.guid()}${middleOnlyCharacter}${chance.guid()}`;
+
+    expect(isValidKey('some-decorator', key)).toBe(true);
+  });
+
+  it('should return true if the key is valid', () => {
+    const key = chance.guid();
+
+    expect(isValidKey('some-decorator', key)).toBe(true);
+  });
+});
diff --git a/src/constants/characterSets.ts b/src/constants/characterSets.ts
index a9a0481..e69b9f3 100644
--- a/src/constants/characterSets.ts
+++ b/src/constants/characterSets.ts
@@ -32,9 +32,9 @@ export const reservedCharacters: ReadonlySet<string> = new Set([
   '\u001F',
 ]);
 
-export const firstCharacterOnly: ReadonlySet<string> = new Set(['\u0040']);
+export const firstOnlyCharacters: ReadonlySet<string> = new Set(['\u0040']);
 
-export const notFirstOrLastCharacter: ReadonlySet<string> = new Set([
+export const middleOnlyCharacters: ReadonlySet<string> = new Set([
   '\u002D',
   '\u005F',
   '\u0020',
diff --git a/src/utils/isValidKey.ts b/src/utils/isValidKey.ts
index 1ebb66c..10fda9e 100644
--- a/src/utils/isValidKey.ts
+++ b/src/utils/isValidKey.ts
@@ -1,6 +1,6 @@
 import {
-  firstCharacterOnly,
-  notFirstOrLastCharacter,
+  firstOnlyCharacters,
+  middleOnlyCharacters,
   reservedCharacters,
 } from '../constants/characterSets';
 
@@ -32,7 +32,7 @@ export const isValidKey = (
   }
 
   const firstCharacterViolations = charactersInKey.filter(
-    (character, index) => firstCharacterOnly.has(character) && index !== 0,
+    (character, index) => firstOnlyCharacters.has(character) && index !== 0,
   );
   if (firstCharacterViolations.length > 0) {
     throw new Error(
@@ -42,7 +42,7 @@ export const isValidKey = (
 
   const firstOrLastCharacterViolations = charactersInKey.filter(
     (character, index) =>
-      notFirstOrLastCharacter.has(character) &&
+      middleOnlyCharacters.has(character) &&
       (index === 0 || index === charactersInKey.length - 1),
   );
   if (firstOrLastCharacterViolations.length > 0) {