diff --git a/packages/type/src/reflection/type.ts b/packages/type/src/reflection/type.ts
index ecde23fe1..b056ebe0b 100644
--- a/packages/type/src/reflection/type.ts
+++ b/packages/type/src/reflection/type.ts
@@ -2332,6 +2332,22 @@ export function getDeepConstructorProperties(type: TypeClass): TypeParameter[] {
     return res;
 }
 
+/**
+ * Returns the index to `type.values` if the given value is part of the enum, exactly or case-insensitive.
+ * Returns -1 if not found.
+ */
+export function getEnumValueIndexMatcher(type: TypeEnum): (value: string | number | undefined | null) => number {
+    const lowerCaseValues = Object.keys(type.enum).map(v => String(v).toLowerCase());
+    return (value): number => {
+        const exactMatch = type.values.indexOf(value);
+        if (exactMatch !== -1) return exactMatch;
+        const lowerCaseMatch = lowerCaseValues.indexOf(String(value).toLowerCase());
+        if (lowerCaseMatch !== -1) return lowerCaseMatch;
+
+        return -1;
+    };
+}
+
 interface StringifyTypeOptions {
     //show type alias names
     showNames: boolean;
diff --git a/packages/type/src/serializer.ts b/packages/type/src/serializer.ts
index a9e95ba9b..446960f87 100644
--- a/packages/type/src/serializer.ts
+++ b/packages/type/src/serializer.ts
@@ -38,6 +38,7 @@ import {
     FindType,
     getConstructorProperties,
     getDeepConstructorProperties,
+    getEnumValueIndexMatcher,
     getTypeJitContainer,
     getTypeObjectLiteralFromTypeClass,
     groupAnnotation,
@@ -1977,17 +1978,10 @@ export class Serializer {
         this.serializeRegistry.register(ReflectionKind.enum, (type, state) => state.addSetter(state.accessor));
         this.deserializeRegistry.register(ReflectionKind.enum, (type, state) => {
             const valuesVar = state.setVariable('values', type.values);
-            const lowercaseNames = state.setVariable('lowercaseNames', Object.keys(type.enum).map(v => v.toLowerCase()));
-            const allLowercased = Object.keys(type.enum).every(v => v.toLowerCase() === v);
-            const enumValues = state.setVariable('enumValues', type.values);
-            const allowLowercase = allLowercased ? '' : `
-                ${state.accessor} = ${enumValues}[${lowercaseNames}.indexOf(String(${state.accessor}).toLowerCase())] ?? ${state.accessor};
-            `;
-
+            const matcher = state.setVariable('enumMatcher', getEnumValueIndexMatcher(type));
             state.addCodeForSetter(`
-                ${allowLowercase}
-                ${state.setter} = ${state.accessor};
-                if (${valuesVar}.indexOf(${state.accessor}) === -1) ${state.throwCode('enum', `'No valid value of ' + ${valuesVar}.join(', ')`)}
+                ${state.setter} = ${valuesVar}[${matcher}(${state.accessor})];
+                if (${valuesVar}.indexOf(${state.setter}) === -1) ${state.throwCode('enum', `'No valid value of ' + ${valuesVar}.join(', ')`)}
             `);
         });
 
@@ -2219,8 +2213,8 @@ export class Serializer {
 
         this.typeGuards.register(1, ReflectionKind.promise, (type, state) => executeTemplates(state, type.type));
         this.typeGuards.register(1, ReflectionKind.enum, (type, state) => {
-            const values = state.setVariable('values', type.values);
-            state.addSetterAndReportErrorIfInvalid('type', 'Invalid enum member', `${values}.indexOf(${state.accessor}) >= 0`);
+            const matcher = state.setVariable('enumMatcher', getEnumValueIndexMatcher(type));
+            state.addSetterAndReportErrorIfInvalid('type', 'Invalid enum member', `${matcher}(${state.accessor}) >= 0`);
         });
         this.typeGuards.register(1, ReflectionKind.array, (type, state) => typeGuardArray(type.type, state));
         this.typeGuards.register(1, ReflectionKind.tuple, typeGuardTuple);
diff --git a/packages/type/tests/serializer.spec.ts b/packages/type/tests/serializer.spec.ts
index a22c4a0f0..f6795b996 100644
--- a/packages/type/tests/serializer.spec.ts
+++ b/packages/type/tests/serializer.spec.ts
@@ -1033,6 +1033,10 @@ test('enum mixed case', () => {
 
     expect(cast<Units>('gram')).toBe('g');
     expect(cast<Units>('gram')).toBe(Units.GRAM);
+
+    expect(cast<number | Units>('GRAM')).toBe(Units.GRAM);
+    expect(cast<number | Units>(23)).toBe(23);
+    expect(cast<number | Units>('Gram')).toBe(Units.GRAM);
 });
 
 test('enum union', () => {