diff --git a/bootstrap-core/src/main/java/de/agilecoders/wicket/core/markup/html/bootstrap/form/FormGroup.java b/bootstrap-core/src/main/java/de/agilecoders/wicket/core/markup/html/bootstrap/form/FormGroup.java
index b1d1c1b02..83ce6bce4 100644
--- a/bootstrap-core/src/main/java/de/agilecoders/wicket/core/markup/html/bootstrap/form/FormGroup.java
+++ b/bootstrap-core/src/main/java/de/agilecoders/wicket/core/markup/html/bootstrap/form/FormGroup.java
@@ -3,6 +3,8 @@
 import de.agilecoders.wicket.core.markup.html.bootstrap.behavior.ICssClassNameProvider;
 import de.agilecoders.wicket.core.util.Attributes;
 import de.agilecoders.wicket.core.util.Components;
+import de.agilecoders.wicket.core.util.CssClassNames;
+
 import org.apache.wicket.AttributeModifier;
 import org.apache.wicket.Component;
 import org.apache.wicket.behavior.AttributeAppender;
@@ -22,6 +24,10 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import static de.agilecoders.wicket.core.util.CssClassNames.Form.controlStateInvalid;
+import static de.agilecoders.wicket.core.util.CssClassNames.Form.controlStateValid;
+import static de.agilecoders.wicket.core.util.CssClassNames.Form.group;
+
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -169,7 +175,7 @@ protected void onComponentTag(ComponentTag tag) {
         super.onComponentTag(tag);
 
         checkComponentTag(tag, "div");
-        Attributes.addClass(tag, "form-group");
+        Attributes.addClass(tag, group);
     }
 
     @Override
@@ -240,16 +246,13 @@ protected void onConfigure() {
         stateClassName = "";
         feedback.setDefaultModelObject("");
 
-        String validComponentClass = "is-valid";
-        String invalidComponentClass = "is-invalid";
-        
         final List<FormComponent<?>> formComponents = findFormComponents();
      	
         formComponents.forEach(c -> c.add(new Behavior() {
 			@Override
 			public void onComponentTag(Component component, ComponentTag tag)
 			{
-				Attributes.removeClass(tag, validComponentClass, invalidComponentClass);
+				Attributes.removeClass(tag, controlStateValid, controlStateInvalid);
 			}
 			
 			@Override
@@ -273,9 +276,9 @@ public boolean isTemporary(Component component) {
         			public void onComponentTag(Component component, ComponentTag tag)
         			{
         				if(FeedbackMessageToCssClassNameTransformer.INVALID_FEEDBACK.equals(stateClassName))
-        					Attributes.addClass(tag, invalidComponentClass);
+        					Attributes.addClass(tag, controlStateInvalid);
         				else
-        					Attributes.addClass(tag, validComponentClass);
+        					Attributes.addClass(tag, controlStateValid);
         			}
         			
         			@Override
diff --git a/bootstrap-core/src/main/java/de/agilecoders/wicket/core/util/CssClassNames.java b/bootstrap-core/src/main/java/de/agilecoders/wicket/core/util/CssClassNames.java
index 189aeffb8..2c068e396 100644
--- a/bootstrap-core/src/main/java/de/agilecoders/wicket/core/util/CssClassNames.java
+++ b/bootstrap-core/src/main/java/de/agilecoders/wicket/core/util/CssClassNames.java
@@ -244,6 +244,8 @@ public static final class Form {
         public static final String form = "form";
         public static final String formRow = "form-row";
         public static final String control = "form-control";
+        public static final String controlStateValid = "is-valid";
+        public static final String controlStateInvalid = "is-invalid";
         public static final String label = "col-form-label";
         public static final String staticControl = "form-control-static";
         public static final String feedbackControl = "form-control-feedback";