diff --git a/flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/functions/StringFunctionsITCase.java b/flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/functions/StringFunctionsITCase.java
index 0fd1c53dac979..076eeb5ba5a33 100644
--- a/flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/functions/StringFunctionsITCase.java
+++ b/flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/functions/StringFunctionsITCase.java
@@ -149,12 +149,15 @@ private Stream<TestSetSpec> bTrimTestCases() {
     private Stream<TestSetSpec> eltTestCases() {
         return Stream.of(
                 TestSetSpec.forFunction(BuiltInFunctionDefinitions.ELT)
-                        .onFieldsWithData(null, null, null, new byte[] {1, 2, 3})
+                        .onFieldsWithData(null, null, null, new byte[] {1, 2, 3}, (byte) 2, (short) 2, 2L)
                         .andDataTypes(
                                 DataTypes.INT(),
                                 DataTypes.STRING(),
                                 DataTypes.BYTES(),
-                                DataTypes.BYTES())
+                                DataTypes.BYTES(),
+                                DataTypes.TINYINT(),
+                                DataTypes.SMALLINT(),
+                                DataTypes.BIGINT())
                         // null input
                         .testResult(
                                 $("f0").elt("a", "b"), "ELT(f0, 'a', 'b')", null, DataTypes.CHAR(1))
@@ -182,6 +185,29 @@ private Stream<TestSetSpec> eltTestCases() {
                                 DataTypes.VARCHAR(5))
                         .testResult(
                                 lit(2).elt("a", "b"), "ELT(2, 'a', 'b')", "b", DataTypes.CHAR(1))
+                        // FLINK-40338: non-INT INTEGER_NUMERIC index must not throw ClassCastException.
+                        // Constant case covers the ExpressionReducer (constant-folding) path;
+                        // field-reference cases below cover the codegen'd operator path.
+                        .testResult(
+                                lit(2).cast(DataTypes.TINYINT()).elt("scala", "java"),
+                                "ELT(CAST(2 AS TINYINT), 'scala', 'java')",
+                                "java",
+                                DataTypes.VARCHAR(5))
+                        .testResult(
+                                $("f4").elt("scala", "java"),
+                                "ELT(f4, 'scala', 'java')",
+                                "java",
+                                DataTypes.VARCHAR(5))
+                        .testResult(
+                                $("f5").elt("scala", "java"),
+                                "ELT(f5, 'scala', 'java')",
+                                "java",
+                                DataTypes.VARCHAR(5))
+                        .testResult(
+                                $("f6").elt("scala", "java"),
+                                "ELT(f6, 'scala', 'java')",
+                                "java",
+                                DataTypes.VARCHAR(5))
                         .testResult(
                                 lit(2).elt($("f2"), $("f3"), $("f3")),
                                 "ELT(2, f2, f3, f3)",
diff --git a/flink-table/flink-table-runtime/src/main/java/org/apache/flink/table/runtime/functions/scalar/EltFunction.java b/flink-table/flink-table-runtime/src/main/java/org/apache/flink/table/runtime/functions/scalar/EltFunction.java
index 9af2cf20367d8..9ad4ac7116dff 100644
--- a/flink-table/flink-table-runtime/src/main/java/org/apache/flink/table/runtime/functions/scalar/EltFunction.java
+++ b/flink-table/flink-table-runtime/src/main/java/org/apache/flink/table/runtime/functions/scalar/EltFunction.java
@@ -40,6 +40,9 @@ public EltFunction(SpecializedContext context) {
         if (idx < 1 || idx > exprs.length) {
             return null;
         }
-        return exprs[(int) index - 1];
+        // Narrow the already-unboxed long instead of casting the Number reference.
+        // Casting `index` (java.lang.Number) to int compiles to a checkcast to Integer
+        // followed by unboxing, which throws ClassCastException for Byte/Short/Long.
+        return exprs[(int) idx - 1];
     }
 }