diff --git a/dist/cjs.js b/dist/cjs.js
--- a/dist/cjs.js
+++ b/dist/cjs.js
@@ -76,11 +76,16 @@ function propertyIsUnsafe(target, key) {
 			&& Object.propertyIsEnumerable.call(target, key)) // and also unsafe if they're nonenumerable.
 }
 
+// Own __proto__ keys must stay data, not replace the result's prototype.
+function setOwnValue(destination, key, value) {
+	Object.defineProperty(destination, key, { value: value, enumerable: true, writable: true, configurable: true })
+}
+
 function mergeObject(target, source, options) {
 	var destination = {};
 	if (options.isMergeableObject(target)) {
 		getKeys(target).forEach(function(key) {
-			destination[key] = cloneUnlessOtherwiseSpecified(target[key], options);
+			setOwnValue(destination, key, cloneUnlessOtherwiseSpecified(target[key], options));
 		});
 	}
 	getKeys(source).forEach(function(key) {
@@ -89,9 +94,9 @@ function mergeObject(target, source, options) {
 		}
 
 		if (propertyIsOnObject(target, key) && options.isMergeableObject(source[key])) {
-			destination[key] = getMergeFunction(key, options)(target[key], source[key], options);
+			setOwnValue(destination, key, getMergeFunction(key, options)(target[key], source[key], options));
 		} else {
-			destination[key] = cloneUnlessOtherwiseSpecified(source[key], options);
+			setOwnValue(destination, key, cloneUnlessOtherwiseSpecified(source[key], options));
 		}
 	});
 	return destination
diff --git a/dist/umd.js b/dist/umd.js
--- a/dist/umd.js
+++ b/dist/umd.js
@@ -80,11 +80,16 @@
 				&& Object.propertyIsEnumerable.call(target, key)) // and also unsafe if they're nonenumerable.
 	}
 
+	// Own __proto__ keys must stay data, not replace the result's prototype.
+	function setOwnValue(destination, key, value) {
+		Object.defineProperty(destination, key, { value: value, enumerable: true, writable: true, configurable: true })
+	}
+
 	function mergeObject(target, source, options) {
 		var destination = {};
 		if (options.isMergeableObject(target)) {
 			getKeys(target).forEach(function(key) {
-				destination[key] = cloneUnlessOtherwiseSpecified(target[key], options);
+				setOwnValue(destination, key, cloneUnlessOtherwiseSpecified(target[key], options));
 			});
 		}
 		getKeys(source).forEach(function(key) {
@@ -93,9 +98,9 @@
 			}
 
 			if (propertyIsOnObject(target, key) && options.isMergeableObject(source[key])) {
-				destination[key] = getMergeFunction(key, options)(target[key], source[key], options);
+				setOwnValue(destination, key, getMergeFunction(key, options)(target[key], source[key], options));
 			} else {
-				destination[key] = cloneUnlessOtherwiseSpecified(source[key], options);
+				setOwnValue(destination, key, cloneUnlessOtherwiseSpecified(source[key], options));
 			}
 		});
 		return destination
diff --git a/index.js b/index.js
--- a/index.js
+++ b/index.js
@@ -51,11 +51,16 @@ function propertyIsUnsafe(target, key) {
 			&& Object.propertyIsEnumerable.call(target, key)) // and also unsafe if they're nonenumerable.
 }
 
+// Own __proto__ keys must stay data, not replace the result's prototype.
+function setOwnValue(destination, key, value) {
+	Object.defineProperty(destination, key, { value: value, enumerable: true, writable: true, configurable: true })
+}
+
 function mergeObject(target, source, options) {
 	var destination = {}
 	if (options.isMergeableObject(target)) {
 		getKeys(target).forEach(function(key) {
-			destination[key] = cloneUnlessOtherwiseSpecified(target[key], options)
+			setOwnValue(destination, key, cloneUnlessOtherwiseSpecified(target[key], options))
 		})
 	}
 	getKeys(source).forEach(function(key) {
@@ -64,9 +69,9 @@ function mergeObject(target, source, options) {
 		}
 
 		if (propertyIsOnObject(target, key) && options.isMergeableObject(source[key])) {
-			destination[key] = getMergeFunction(key, options)(target[key], source[key], options)
+			setOwnValue(destination, key, getMergeFunction(key, options)(target[key], source[key], options))
 		} else {
-			destination[key] = cloneUnlessOtherwiseSpecified(source[key], options)
+			setOwnValue(destination, key, cloneUnlessOtherwiseSpecified(source[key], options))
 		}
 	})
 	return destination