diff --git a/src/pypromice/process/aws.py b/src/pypromice/process/aws.py
index 1d203ad4..e1285fee 100644
--- a/src/pypromice/process/aws.py
+++ b/src/pypromice/process/aws.py
@@ -94,7 +94,7 @@ def __init__(
         formats = {dataset.attrs["format"].lower() for dataset in self.L0}
         if "raw" in formats:
             self.format = "raw"
-        elif "STM" in formats:
+        elif "stm" in formats:
             self.format = "STM"
         elif "tx" in formats:
             self.format = "tx"