diff --git a/panda/src/physics/physicsObject.I b/panda/src/physics/physicsObject.I
index 5840d23e49..c2bd4d07f8 100644
--- a/panda/src/physics/physicsObject.I
+++ b/panda/src/physics/physicsObject.I
@@ -23,6 +23,7 @@
 ////////////////////////////////////////////////////////////////////
 INLINE void PhysicsObject::
 set_mass(float m) {
+  nassertv(m > 0);
   _mass = m;
 }