diff --git a/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/InputGuardrailPromptTemplateTest.java b/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/InputGuardrailPromptTemplateTest.java
index 92ff09b53..921d77b0c 100644
--- a/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/InputGuardrailPromptTemplateTest.java
+++ b/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/InputGuardrailPromptTemplateTest.java
@@ -142,7 +142,7 @@ void shouldWorkWithMemoryIdAndOneItemFromList() {
     void shouldWorkWithNoUserMessage() {
         // UserMessage annotation is not provided, then no user message template should be available
         aiService.saySomething("Is this a parameter or a prompt?");
-        assertThat(guardrailValidation.spyUserMessageTemplate()).isNull();
+        assertThat(guardrailValidation.spyUserMessageTemplate()).isEmpty();
         assertThat(guardrailValidation.spyVariables()).isEmpty();
     }
 
diff --git a/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/OutputGuardrailPromptTemplateTest.java b/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/OutputGuardrailPromptTemplateTest.java
index 3c170f17b..1bf2ea7d4 100644
--- a/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/OutputGuardrailPromptTemplateTest.java
+++ b/core/deployment/src/test/java/io/quarkiverse/langchain4j/test/guardrails/OutputGuardrailPromptTemplateTest.java
@@ -139,7 +139,7 @@ void shouldWorkWithMemoryIdAndOneItemFromList() {
     void shouldWorkWithNoUserMessage() {
         // UserMessage annotation is not provided, then no user message template should be available
         aiService.saySomething("Is this a parameter or a prompt?");
-        assertThat(guardrailValidation.spyUserMessageTemplate()).isNull();
+        assertThat(guardrailValidation.spyUserMessageTemplate()).isEmpty();
         assertThat(guardrailValidation.spyVariables()).isEmpty();
     }
 
diff --git a/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/InputGuardrailParams.java b/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/InputGuardrailParams.java
index b27371263..1900d27b1 100644
--- a/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/InputGuardrailParams.java
+++ b/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/InputGuardrailParams.java
@@ -12,8 +12,8 @@
  * @param userMessage the user message, cannot be {@code null}
  * @param memory the memory, can be {@code null} or empty
  * @param augmentationResult the augmentation result, can be {@code null}
- * @param userMessageTemplate the user message template, can be {@code null} when @UserMessage is not provided.
- * @param variables the variable to be used with userMessageTemplate, can be {@code null} or empty
+ * @param userMessageTemplate the user message template, cannot be {@code null}
+ * @param variables the variable to be used with userMessageTemplate, cannot be {@code null}
  */
 public record InputGuardrailParams(UserMessage userMessage, ChatMemory memory,
         AugmentationResult augmentationResult, String userMessageTemplate,
diff --git a/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/OutputGuardrailParams.java b/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/OutputGuardrailParams.java
index 3bc39f2cd..0162c5f5a 100644
--- a/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/OutputGuardrailParams.java
+++ b/core/runtime/src/main/java/io/quarkiverse/langchain4j/guardrails/OutputGuardrailParams.java
@@ -12,8 +12,8 @@
  * @param responseFromLLM the response from the LLM
  * @param memory the memory, can be {@code null} or empty
  * @param augmentationResult the augmentation result, can be {@code null}
- * @param userMessageTemplate the user message template, can be {@code null} when @UserMessage is not provided.
- * @param variables the variable to be used with userMessageTemplate, can be {@code null} or empty
+ * @param userMessageTemplate the user message template, cannot be {@code null}
+ * @param variables the variable to be used with userMessageTemplate, cannot be {@code null}
  */
 public record OutputGuardrailParams(AiMessage responseFromLLM, ChatMemory memory,
         AugmentationResult augmentationResult, String userMessageTemplate,
diff --git a/core/runtime/src/main/java/io/quarkiverse/langchain4j/runtime/aiservice/AiServiceMethodCreateInfo.java b/core/runtime/src/main/java/io/quarkiverse/langchain4j/runtime/aiservice/AiServiceMethodCreateInfo.java
index dc095ec2b..e1c26af15 100644
--- a/core/runtime/src/main/java/io/quarkiverse/langchain4j/runtime/aiservice/AiServiceMethodCreateInfo.java
+++ b/core/runtime/src/main/java/io/quarkiverse/langchain4j/runtime/aiservice/AiServiceMethodCreateInfo.java
@@ -1,5 +1,7 @@
 package io.quarkiverse.langchain4j.runtime.aiservice;
 
+import static org.apache.commons.lang3.StringUtils.EMPTY;
+
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Map;
@@ -197,7 +199,7 @@ public String getUserMessageTemplate() {
         Optional<String> userMessageTemplateOpt = this.getUserMessageInfo().template()
                 .flatMap(AiServiceMethodCreateInfo.TemplateInfo::text);
 
-        return userMessageTemplateOpt.orElse(null);
+        return userMessageTemplateOpt.orElse(EMPTY);
     }
 
     public record UserMessageInfo(Optional<TemplateInfo> template,