diff --git a/src/sharpness/metrics.py b/src/sharpness/metrics.py
index 46d5e73..4e03a2f 100644
--- a/src/sharpness/metrics.py
+++ b/src/sharpness/metrics.py
@@ -21,7 +21,7 @@
 
 def rmse(X, T):
     """Bivariate -- Root Mean Squared Error"""
-    return np.sqrt(np.mean(X - T) ** 2)
+    return np.sqrt(np.mean((X - T) ** 2))
 
 
 def ssim(X, T, win_size=7, data_range=255):