diff --git a/flatlaf-core/build.gradle.kts b/flatlaf-core/build.gradle.kts
index b9630ac0..e05b5642 100644
--- a/flatlaf-core/build.gradle.kts
+++ b/flatlaf-core/build.gradle.kts
@@ -66,6 +66,9 @@ tasks {
 	test {
 		useJUnitPlatform()
 		testLogging.exceptionFormat = org.gradle.api.tasks.testing.logging.TestExceptionFormat.FULL
+
+		if( JavaVersion.current() >= JavaVersion.VERSION_1_9 )
+			jvmArgs( listOf( "--add-opens", "java.desktop/javax.swing.plaf.basic=ALL-UNNAMED" ) )
 	}
 
 	register( "sigtestGenerate" ) {
diff --git a/flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableValue.java b/flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableValue.java
index 97efbb4b..c105f2e7 100644
--- a/flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableValue.java
+++ b/flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableValue.java
@@ -25,6 +25,7 @@ import java.awt.Font;
 import java.awt.Graphics;
 import java.awt.Insets;
 import java.lang.reflect.Method;
+import java.util.Locale;
 import javax.swing.AbstractButton;
 import javax.swing.Icon;
 import javax.swing.JButton;
@@ -108,7 +109,7 @@ public class TestFlatStyleableValue
 	}
 
 	private void testFloat( JComponent c, StyleableUI ui, String key, float value ) {
-		applyStyle( c, ui, String.format( "%s: %f", key, value ) );
+		applyStyle( c, ui, String.format( Locale.ENGLISH, "%s: %f", key, value ) );
 		assertEquals( value, ui.getStyleableValue( c, key ) );
 	}