diff --git a/broker/jfjoch_api.yaml b/broker/jfjoch_api.yaml
index 68f25aefb..7f0b0324e 100644
--- a/broker/jfjoch_api.yaml
+++ b/broker/jfjoch_api.yaml
@@ -480,6 +480,7 @@ components:
         total_flux:
           type: number
           format: float
+          minimum: 0.0
           description: |
             /entry/beam/total_flux in NXmx
             Flux incident on beam plane in photons per second. In other words this is the flux integrated over area. [photons/s]