diff --git a/.github/workflows/quicktest.yml b/.github/workflows/quicktest.yml
index b0bd62620..1d78a0795 100644
--- a/.github/workflows/quicktest.yml
+++ b/.github/workflows/quicktest.yml
@@ -29,6 +29,15 @@ jobs:
           php-version: ${{ matrix.php }}
           coverage: none
 
+      # At least one test needs a non-en_US locale to be available, so make sure it is.
+      - name: Install locales
+        run: |
+          sudo apt-get update
+          sudo apt-get install locales-all
+
+      - name: Show available locales
+        run: locale -a
+
       # Install dependencies and handle caching in one go.
       # @link https://github.com/marketplace/actions/install-composer-dependencies
       - name: Install Composer dependencies - normal
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 773f8cd66..62dfe38a1 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -61,6 +61,15 @@ jobs:
           coverage: ${{ steps.set_cov.outputs.COV }}
           tools: cs2pr
 
+      # At least one test needs a non-en_US locale to be available, so make sure it is.
+      - name: Install locales
+        run: |
+          sudo apt-get update
+          sudo apt-get install locales-all
+
+      - name: Show available locales
+        run: locale -a
+
       # Install dependencies and handle caching in one go.
       # @link https://github.com/marketplace/actions/install-composer-dependencies
       - name: Install Composer dependencies - normal
diff --git a/tests/Transport/FsockopenTest.php b/tests/Transport/FsockopenTest.php
index 5af8e7639..ba0f03be2 100644
--- a/tests/Transport/FsockopenTest.php
+++ b/tests/Transport/FsockopenTest.php
@@ -53,4 +53,33 @@ public function testContentLengthHeader() {
 	public function checkContentLengthHeader($headers) {
 		$this->assertStringContainsString('Content-Length: 0', $headers);
 	}
+
+	/**
+	 * Issue #335/#339.
+	 */
+	public function testHTTPVersionHeader() {
+		// Remember the original locale.
+		$locale = setlocale(LC_NUMERIC, 0);
+
+		// Set the locale to one using commas for the decimal point.
+		setlocale(LC_NUMERIC, 'de_DE@euro', 'de_DE.utf8', 'de_DE', 'de', 'ge');
+
+		// Make sure the locale was changed.
+		$this->assertNotSame($locale, setlocale(LC_NUMERIC, 0), 'Changing the locale failed');
+
+		$hooks = new Requests_Hooks();
+		$hooks->register('fsockopen.after_headers', array($this, 'checkHTTPVersionHeader'));
+
+		Requests::post(httpbin('/post'), array(), array(), $this->getOptions(array('hooks' => $hooks)));
+
+		// Reset the locale.
+		setlocale(LC_NUMERIC, $locale);
+	}
+
+	/**
+	 * Issue #335/#339.
+	 */
+	public function checkHTTPVersionHeader($headers) {
+		$this->assertStringContainsString('HTTP/1.1', $headers, 'HTTP header is influenced by the system locale');
+	}
 }