diff --git a/apps/dav/lib/Connector/Sabre/File.php b/apps/dav/lib/Connector/Sabre/File.php
index c2c6baaa4893..7b2c405c18f3 100644
--- a/apps/dav/lib/Connector/Sabre/File.php
+++ b/apps/dav/lib/Connector/Sabre/File.php
@@ -232,7 +232,7 @@ public function put($data) {
 			if (isset($this->request->server['HTTP_X_OC_MTIME'])) {
 				$mtime = $this->sanitizeMtime($this->request->server ['HTTP_X_OC_MTIME']);
 				if ($this->fileView->touch($this->path, $mtime)) {
-					header('X-OC-MTime: accepted');
+					$this->header('X-OC-MTime: accepted');
 				}
 			}
 			
@@ -498,7 +498,7 @@ private function createFileChunked($data) {
 						$this->request->server ['HTTP_X_OC_MTIME']
 					);
 					if ($targetStorage->touch($targetInternalPath, $mtime)) {
-						header('X-OC-MTime: accepted');
+						$this->header('X-OC-MTime: accepted');
 					}
 				}
 
@@ -651,4 +651,8 @@ public function getChecksum($algo = null) {
 
 		return '';
 	}
+
+	protected function header($string) {
+		\header($string);
+	}
 }
diff --git a/apps/dav/tests/unit/Connector/Sabre/FileTest.php b/apps/dav/tests/unit/Connector/Sabre/FileTest.php
index 7e78ffd49905..b665cb3273d0 100644
--- a/apps/dav/tests/unit/Connector/Sabre/FileTest.php
+++ b/apps/dav/tests/unit/Connector/Sabre/FileTest.php
@@ -325,7 +325,11 @@ private function doPut($path, $viewRoot = null, \OC\AppFramework\Http\Request $r
 			null
 		);
 
-		$file = new File($view, $info, null, $request);
+		/** @var File | \PHPUnit_Framework_MockObject_MockObject $file */
+		$file = $this->getMockBuilder(File::class)
+			->setConstructorArgs([$view, $info, null, $request])
+			->setMethods(['header'])
+			->getMock();
 
 		// beforeMethod locks
 		$view->lockFile($path, ILockingProvider::LOCK_SHARED);
@@ -345,15 +349,6 @@ public function testPutSingleFile() {
 		$this->assertNotEmpty($this->doPut('/foo.txt'));
 	}
 
-	/**
-	 * Determine if the underlying storage supports a negative mtime value
-	 *
-	 * @return boolean true if negative mtime is supported
-	 */
-	private function supportsNegativeMtime() {
-		return (getenv("PRIMARY_STORAGE_CONFIG") !== "swift");
-	}
-
 	public function legalMtimeProvider() {
 		return [
 			"string" => [
@@ -398,19 +393,17 @@ public function legalMtimeProvider() {
 			],
 			"negative int" => [
 					'HTTP_X_OC_MTIME' => -34,
-					'expected result' => ($this->supportsNegativeMtime() ? -34 : 0)
+					'expected result' => -34
 			],
 			"negative float" => [
 					'HTTP_X_OC_MTIME' => -34.43,
-					'expected result' => ($this->supportsNegativeMtime() ? -34 : 0)
+					'expected result' => -34
 			],
 		];
 	}
 
 	/**
 	 * Test putting a file with string Mtime
-	 * @runInSeparateProcess
-	 * @preserveGlobalState disabled
 	 * @dataProvider legalMtimeProvider
 	 */
 	public function testPutSingleFileLegalMtime($requestMtime, $resultMtime) {
@@ -426,8 +419,6 @@ public function testPutSingleFileLegalMtime($requestMtime, $resultMtime) {
 
 	/**
 	 * Test putting a file with string Mtime using chunking
-	 * @runInSeparateProcess
-	 * @preserveGlobalState disabled
 	 * @dataProvider legalMtimeProvider
 	 */
 	public function testChunkedPutLegalMtime($requestMtime, $resultMtime) {
diff --git a/lib/private/Files/Storage/Common.php b/lib/private/Files/Storage/Common.php
index c0c6af2e49de..1c56606f9610 100644
--- a/lib/private/Files/Storage/Common.php
+++ b/lib/private/Files/Storage/Common.php
@@ -170,7 +170,7 @@ public function getPermissions($path) {
 
 	public function filemtime($path) {
 		$stat = $this->stat($path);
-		if (isset($stat['mtime']) && $stat['mtime'] > 0) {
+		if (isset($stat['mtime'])) {
 			return $stat['mtime'];
 		} else {
 			return 0;