diff --git a/docs/changelog/131917.yaml b/docs/changelog/131917.yaml
new file mode 100644
index 0000000000000..9d69e1653e711
--- /dev/null
+++ b/docs/changelog/131917.yaml
@@ -0,0 +1,6 @@
+pr: 131917
+summary: Fix NPE on empty to_lower/to_upper call
+area: ES|QL
+type: bug
+issues:
+ - 131913
diff --git a/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistry.java b/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistry.java
index 06994c5c2ff8b..139c3d09e703e 100644
--- a/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistry.java
+++ b/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistry.java
@@ -1068,10 +1068,10 @@ public static <T extends Function> FunctionDefinition def(
         String... names
     ) {
         FunctionBuilder builder = (source, children, cfg) -> {
-            if (children.size() > 1) {
+            if (children.size() != 1) {
                 throw new QlIllegalArgumentException("expects exactly one argument");
             }
-            Expression ex = children.size() == 1 ? children.get(0) : null;
+            Expression ex = children.get(0);
             return ctorRef.build(source, ex, cfg);
         };
         return def(function, builder, names);
diff --git a/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistryTests.java b/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistryTests.java
index 50cbbdf4a9338..5c2fc3aace2ec 100644
--- a/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistryTests.java
+++ b/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/EsqlFunctionRegistryTests.java
@@ -29,6 +29,7 @@
 import static org.elasticsearch.xpack.esql.ConfigurationTestUtils.randomConfiguration;
 import static org.elasticsearch.xpack.esql.expression.function.EsqlFunctionRegistry.def;
 import static org.elasticsearch.xpack.esql.expression.function.FunctionResolutionStrategy.DEFAULT;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.endsWith;
 import static org.hamcrest.Matchers.is;
 import static org.mockito.Mockito.mock;
@@ -159,6 +160,9 @@ public void testConfigurationOptionalFunction() {
         );
         def = r.resolveFunction(r.resolveAlias("DUMMY"));
         assertEquals(ur.source(), ur.buildResolved(randomConfiguration(), def).source());
+
+        ParsingException e = expectThrows(ParsingException.class, () -> uf(DEFAULT).buildResolved(randomConfiguration(), def));
+        assertThat(e.getMessage(), containsString("expects exactly one argument"));
     }
 
     private static UnresolvedFunction uf(FunctionResolutionStrategy resolutionStrategy, Expression... children) {