diff --git a/src/cogserver/algorithms/rca.py b/src/cogserver/algorithms/rca.py
index 8a56a23..d1361c7 100644
--- a/src/cogserver/algorithms/rca.py
+++ b/src/cogserver/algorithms/rca.py
@@ -80,7 +80,7 @@ def __call__(self, img: ImageData) -> ImageData:
         #v = data.ptp()*.1
         datam = (data > -v) & (data < v)
         if self.threshold:
-            datam |= np.abs(data)< self.threshold
+            datam = datam | np.abs(data)< self.threshold
         arr = numpy.ma.masked_array(data*100, dtype=self.output_dtype, mask=valid_mask | datam )
         return ImageData(
             arr,