diff --git a/src/main/java/it/cavallium/data/generator/SourcesGenerator.java b/src/main/java/it/cavallium/data/generator/SourcesGenerator.java
index 7e997e7..1ecdf7f 100644
--- a/src/main/java/it/cavallium/data/generator/SourcesGenerator.java
+++ b/src/main/java/it/cavallium/data/generator/SourcesGenerator.java
@@ -1475,7 +1475,7 @@ public class SourcesGenerator {
 										Family toFamily = typeFamily.get(toTypeName);
 										TypeName toType = nextVersionTypeTypes.get(toTypeName);
 										Objects.requireNonNull(toType,
-												"Type " + toTypeName + " is unknown in version " + nextVersion.orElse(null)
+												"Type " + key + "." + toTypeName + " is unknown in version " + nextVersion.orElse(null)
 										);
 										TypeName toTypeBoxed = toType.isPrimitive() ? toType.box() : toType;
 										{