diff --git a/ci/checkstyle/checkstyle.xml b/ci/checkstyle/checkstyle.xml
index 3bd7a11..311f255 100644
--- a/ci/checkstyle/checkstyle.xml
+++ b/ci/checkstyle/checkstyle.xml
@@ -90,6 +90,10 @@
     <module name="UnusedImports">
       <property name="processJavadoc" value="true"/>
     </module>
+    <module name="UpperEll">
+      <!-- Checks that long constants are defined with an upper ell.-->
+      <property name="severity" value="error"/>
+    </module>
     <module name="WhitespaceAfter">
       <!-- Checks that commas, semicolons and typecasts are followed by
            whitespace.