diff --git a/smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/stdlib/Substring.java b/smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/stdlib/Substring.java
index 95f756162f1..275a1a0b2b5 100644
--- a/smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/stdlib/Substring.java
+++ b/smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/stdlib/Substring.java
@@ -44,7 +44,7 @@ public List<Type> getArguments() {
 
     @Override
     public Type getReturnType() {
-        return Type.string();
+        return Type.optional(Type.string());
     }
 
     @Override