diff --git a/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java b/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java
index 3d21f9f8a..65551d657 100644
--- a/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java
+++ b/org.eclipse.jgit.junit/src/org/eclipse/jgit/junit/MockSystemReader.java
@@ -184,6 +184,7 @@ public void setWindows() {
 		setProperty("file.separator", "\\");
 		setProperty("path.separator", ";");
 		setProperty("line.separator", "\r\n");
+		setPlatformChecker();
 	}
 
 	/**
@@ -194,5 +195,6 @@ public void setUnix() {
 		setProperty("file.separator", "/");
 		setProperty("path.separator", ":");
 		setProperty("line.separator", "\n");
+		setPlatformChecker();
 	}
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java
index 3447f639d..f6f415e85 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/SystemReader.java
@@ -163,11 +163,19 @@ public static void setInstance(SystemReader newReader) {
 	private void init() {
 		// Creating ObjectChecker must be deferred. Unit tests change
 		// behavior of is{Windows,MacOS} in constructor of subclass.
-		if (platformChecker == null) {
-			platformChecker = new ObjectChecker()
-				.setSafeForWindows(isWindows())
-				.setSafeForMacOS(isMacOS());
-		}
+		if (platformChecker == null)
+			setPlatformChecker();
+	}
+
+	/**
+	 * Should be used in tests when the platform is explicitly changed.
+	 *
+	 * @since 3.6
+	 */
+	protected final void setPlatformChecker() {
+		platformChecker = new ObjectChecker()
+			.setSafeForWindows(isWindows())
+			.setSafeForMacOS(isMacOS());
 	}
 
 	/**