diff --git a/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/predicate/operator/arithmetic/EsqlArithmeticOperation.java b/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/predicate/operator/arithmetic/EsqlArithmeticOperation.java
index b58b7eb51fe7e..0dd3ea2a80278 100644
--- a/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/predicate/operator/arithmetic/EsqlArithmeticOperation.java
+++ b/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/predicate/operator/arithmetic/EsqlArithmeticOperation.java
@@ -21,6 +21,7 @@
 import org.elasticsearch.xpack.ql.type.DataTypes;
 
 import java.io.IOException;
+import java.util.Locale;
 import java.util.function.Function;
 
 import static org.elasticsearch.common.logging.LoggerMessageFormat.format;
@@ -149,8 +150,8 @@ static String formatIncombatibleTypesMessage(String symbol, String leftType, Str
             null,
             "[{}] has arguments with incompatible types [{}] and [{}]",
             symbol,
-            leftType.toUpperCase(),
-            rightType.toUpperCase()
+            leftType.toUpperCase(Locale.ROOT),
+            rightType.toUpperCase(Locale.ROOT)
         );
     }