diff --git a/processor/src/main/java/org/mapstruct/ap/spi/CaseEnumTransformationStrategy.java b/processor/src/main/java/org/mapstruct/ap/spi/CaseEnumTransformationStrategy.java
index 2f99f56ce..3a4ba18ad 100644
--- a/processor/src/main/java/org/mapstruct/ap/spi/CaseEnumTransformationStrategy.java
+++ b/processor/src/main/java/org/mapstruct/ap/spi/CaseEnumTransformationStrategy.java
@@ -20,6 +20,7 @@ public class CaseEnumTransformationStrategy implements EnumTransformationStrateg
     private static final String UPPER = "upper";
     private static final String LOWER = "lower";
     private static final String CAPITAL = "capital";
+    private static final String CASE_ENUM_TRANSFORMATION_STRATEGIES = UPPER + ", " + LOWER + ", " + CAPITAL;
 
     @Override
     public String getStrategyName() {
@@ -37,7 +38,8 @@ public class CaseEnumTransformationStrategy implements EnumTransformationStrateg
                 return capitalize( value );
             default:
                 throw new IllegalArgumentException(
-                    "Unexpected configuration for enum case transformation: " + configuration );
+                    "Unexpected configuration for enum case transformation: " + configuration +
+                    ". Allowed values: " + CASE_ENUM_TRANSFORMATION_STRATEGIES);
         }
     }
 
diff --git a/processor/src/test/java/org/mapstruct/ap/test/value/nametransformation/EnumNameTransformationStrategyTest.java b/processor/src/test/java/org/mapstruct/ap/test/value/nametransformation/EnumNameTransformationStrategyTest.java
index 7a29ae40a..012fc9961 100644
--- a/processor/src/test/java/org/mapstruct/ap/test/value/nametransformation/EnumNameTransformationStrategyTest.java
+++ b/processor/src/test/java/org/mapstruct/ap/test/value/nametransformation/EnumNameTransformationStrategyTest.java
@@ -112,7 +112,8 @@ public class EnumNameTransformationStrategyTest {
                 kind = javax.tools.Diagnostic.Kind.ERROR,
                 line = 18,
                 message = "Illegal transformation for 'case' EnumTransformationStrategy." +
-                    " Error: 'Unexpected configuration for enum case transformation: unknown'."
+                    " Error: 'Unexpected configuration for enum case transformation: unknown." +
+                    " Allowed values: upper, lower, capital'."
             )
         }
     )