diff --git a/src/properties-minifier.ts b/src/properties-minifier.ts
index 57f59aa..fbbf1fa 100644
--- a/src/properties-minifier.ts
+++ b/src/properties-minifier.ts
@@ -201,7 +201,7 @@ function isPrivateNonStaticClassMember(symbol: ts.Symbol | undefined): boolean {
 
 	return symbol.declarations.some((x: ts.Declaration) => {
 		// terser / uglify property minifiers aren't able to handle decorators
-		return (isClassMember(x) && !hasDecorators(x) || isConstructorParameter(x)) && isPrivateNonStatic(x);
+		return (isClassMember(x) && !hasDecorators(x) || isConstructorParameter(x)) && isPrivateNonStatic(x) && !hasModifier(x, ts.SyntaxKind.DeclareKeyword);
 	});
 }
 
diff --git a/tests/functional-test-cases.ts b/tests/functional-test-cases.ts
index 7a6db9e..937fe00 100644
--- a/tests/functional-test-cases.ts
+++ b/tests/functional-test-cases.ts
@@ -73,6 +73,44 @@ function checkDiagnosticsErrors(diagnostics: ReadonlyArray<ts.Diagnostic>): void
 	assert.strictEqual(diagnostics.length, 0, ts.formatDiagnostics(diagnostics, formatDiagnosticsHost).trim());
 }
 
+const enum Constants {
+	NoInputsWereFoundDiagnosticCode = 18003,
+}
+
+const parseConfigHost: ts.ParseConfigHost = {
+	useCaseSensitiveFileNames: ts.sys.useCaseSensitiveFileNames,
+	readDirectory: ts.sys.readDirectory,
+	fileExists: ts.sys.fileExists,
+	readFile: ts.sys.readFile,
+};
+
+function getCompilerOptions(configFileName: string): ts.CompilerOptions | null {
+	if (!fs.existsSync(configFileName)) {
+		return null;
+	}
+
+	const configParseResult = ts.readConfigFile(configFileName, ts.sys.readFile);
+
+	checkDiagnosticsErrors(configParseResult.error !== undefined ? [configParseResult.error] : []);
+
+	const compilerOptionsParseResult = ts.parseJsonConfigFileContent(
+		configParseResult.config,
+		parseConfigHost,
+		path.resolve(path.dirname(configFileName)),
+		undefined,
+		configFileName
+	);
+
+	// we don't want to raise an error if no inputs found in a config file
+	// because this error is mostly for CLI, but we'll pass an inputs in createProgram
+	const diagnostics = compilerOptionsParseResult.errors
+		.filter((d: ts.Diagnostic) => d.code !== Constants.NoInputsWereFoundDiagnosticCode);
+
+	checkDiagnosticsErrors(diagnostics);
+
+	return compilerOptionsParseResult.options;
+}
+
 describe('Functional tests', () => {
 	for (const testCase of getTestCases()) {
 		it(testCase.name, () => {
@@ -80,7 +118,7 @@ describe('Functional tests', () => {
 				rootNames: [testCase.inputFileName],
 				options: {
 					target: ts.ScriptTarget.ES5,
-					experimentalDecorators: true,
+					...getCompilerOptions(path.join(testCase.inputFileName, '..', 'tsconfig.json')),
 				},
 			});
 
diff --git a/tests/test-cases/useDefineForClassFields-and-declare/input.ts b/tests/test-cases/useDefineForClassFields-and-declare/input.ts
new file mode 100644
index 0000000..47c4f37
--- /dev/null
+++ b/tests/test-cases/useDefineForClassFields-and-declare/input.ts
@@ -0,0 +1,9 @@
+class Class {
+	public declare publicField: number;
+	protected declare protectedField: number;
+	private declare privateField: number;
+
+	public method() {
+		console.log(this.publicField, this.protectedField, this.privateField);
+	}
+}
diff --git a/tests/test-cases/useDefineForClassFields-and-declare/output.js b/tests/test-cases/useDefineForClassFields-and-declare/output.js
new file mode 100644
index 0000000..4a34ac0
--- /dev/null
+++ b/tests/test-cases/useDefineForClassFields-and-declare/output.js
@@ -0,0 +1,13 @@
+var Class = /** @class */ (function () {
+    function Class() {
+    }
+    Object.defineProperty(Class.prototype, "method", {
+        enumerable: false,
+        configurable: true,
+        writable: true,
+        value: function () {
+            console.log(this.publicField, this.protectedField, this.privateField);
+        }
+    });
+    return Class;
+}());
diff --git a/tests/test-cases/useDefineForClassFields-and-declare/tsconfig.json b/tests/test-cases/useDefineForClassFields-and-declare/tsconfig.json
new file mode 100644
index 0000000..bb04b29
--- /dev/null
+++ b/tests/test-cases/useDefineForClassFields-and-declare/tsconfig.json
@@ -0,0 +1,6 @@
+{
+	"compilerOptions": {
+		"target": "es5",
+		"useDefineForClassFields": true
+	}
+}