diff --git a/examples/rfmip-clear-sky/compare-to-reference.py b/examples/rfmip-clear-sky/compare-to-reference.py
index de6cc05bf..3f7b0c629 100755
--- a/examples/rfmip-clear-sky/compare-to-reference.py
+++ b/examples/rfmip-clear-sky/compare-to-reference.py
@@ -37,6 +37,9 @@
 
     failed = False
     for v in variables:
+        if np.any(np.isnan(ref.variables[v].values)):
+            raise Exception(
+                v + ": some ref values are missing. Now that is strange.")
         if np.all(np.isnan(tst.variables[v].values)):
             raise Exception(
                 v + ": all test values are missing. Were the tests run?")