diff --git a/spine-haxe/spine-haxe/spine/Slider.hx b/spine-haxe/spine-haxe/spine/Slider.hx
index 1fdd39074..e5b73c590 100644
--- a/spine-haxe/spine-haxe/spine/Slider.hx
+++ b/spine-haxe/spine-haxe/spine/Slider.hx
@@ -117,8 +117,11 @@ class Slider extends Constraint<Slider, SliderData, SliderPose> {
 						skeleton.constrained(physics[ii]);
 				} else
 					skeleton.constrained(constraints[timeline.constraintIndex]);
-			} else if (Std.isOfType(t, ConstraintTimeline)) //
-				skeleton.constrained(constraints[cast(t, ConstraintTimeline).getConstraintIndex()]);
+			} else if (Std.isOfType(t, ConstraintTimeline)) {
+				var constraintIndex = cast(t, ConstraintTimeline).getConstraintIndex();
+				if (constraintIndex != -1)
+					skeleton.constrained(constraints[constraintIndex]);
+			}
 		}
 	}
 }
diff --git a/spine-haxe/spine-haxe/spine/animation/ConstraintTimeline.hx b/spine-haxe/spine-haxe/spine/animation/ConstraintTimeline.hx
index 751e243d2..38fb973ad 100644
--- a/spine-haxe/spine-haxe/spine/animation/ConstraintTimeline.hx
+++ b/spine-haxe/spine-haxe/spine/animation/ConstraintTimeline.hx
@@ -30,6 +30,7 @@
 package spine.animation;
 
 interface ConstraintTimeline {
-	/** The index of the constraint in spine.Skeleton.constraints that will be changed when this timeline is applied. */
+	/** The index of the constraint in spine.Skeleton.constraints that will be changed when this timeline is applied, or
+	 * -1 if a specific constraint will not be changed. */
 	public function getConstraintIndex():Int;
 }