diff --git a/paimon-common/src/main/java/org/apache/paimon/predicate/SubstringTransform.java b/paimon-common/src/main/java/org/apache/paimon/predicate/SubstringTransform.java
index 00431a61f3ff..054422a20125 100644
--- a/paimon-common/src/main/java/org/apache/paimon/predicate/SubstringTransform.java
+++ b/paimon-common/src/main/java/org/apache/paimon/predicate/SubstringTransform.java
@@ -56,7 +56,8 @@ public final Object transform(InternalRow row) {
         if (source instanceof FieldRef) {
             FieldRef sourceFieldRef = (FieldRef) source;
             checkArgument(sourceFieldRef.type().is(CHARACTER_STRING));
-            sourceString = row.isNullAt(0) ? null : row.getString(sourceFieldRef.index());
+            int sourceIndex = sourceFieldRef.index();
+            sourceString = row.isNullAt(sourceIndex) ? null : row.getString(sourceIndex);
         } else {
             sourceString = (BinaryString) inputs.get(0);
         }
diff --git a/paimon-common/src/test/java/org/apache/paimon/predicate/SubstringTransformTest.java b/paimon-common/src/test/java/org/apache/paimon/predicate/SubstringTransformTest.java
index 8ddb74440b7c..b4d998bea9ec 100644
--- a/paimon-common/src/test/java/org/apache/paimon/predicate/SubstringTransformTest.java
+++ b/paimon-common/src/test/java/org/apache/paimon/predicate/SubstringTransformTest.java
@@ -99,4 +99,17 @@ public void testSubstringRefInputs() {
                                 3));
         assertThat(result).isEqualTo(BinaryString.fromString("ell"));
     }
+
+    @Test
+    public void testSubstringRefInputUsesSourceFieldNullability() {
+        List<Object> inputs = new ArrayList<>();
+        inputs.add(new FieldRef(1, "f1", DataTypes.STRING()));
+        inputs.add(2);
+        inputs.add(3);
+        SubstringTransform transform = new SubstringTransform(inputs);
+
+        Object result = transform.transform(GenericRow.of(null, BinaryString.fromString("hello")));
+
+        assertThat(result).isEqualTo(BinaryString.fromString("ell"));
+    }
 }