Index: src/main/java/org/apache/logging/log4j/message/StructuredDataMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/StructuredDataMessage.java	(revision 1408496)
+++ src/main/java/org/apache/logging/log4j/message/StructuredDataMessage.java	(working copy)
@@ -190,8 +190,8 @@
 
     @Override
     protected void validate(String key, String value) {
-        if (value.length() > MAX_LENGTH) {
-            throw new IllegalArgumentException("Structured data values are limited to 32 characters. key: " + key +
+        if (key.length() > MAX_LENGTH) {
+            throw new IllegalArgumentException("Structured data keys are limited to 32 characters. key: " + key +
                 " value: " + value);
         }
     }