diff --git a/libraries/tools/kotlin-maven-plugin/src/main/java/org/jetbrains/kotlin/maven/kapt/KaptJVMCompilerMojo.java b/libraries/tools/kotlin-maven-plugin/src/main/java/org/jetbrains/kotlin/maven/kapt/KaptJVMCompilerMojo.java
index a334c7094c5..4af6b7ac787 100644
--- a/libraries/tools/kotlin-maven-plugin/src/main/java/org/jetbrains/kotlin/maven/kapt/KaptJVMCompilerMojo.java
+++ b/libraries/tools/kotlin-maven-plugin/src/main/java/org/jetbrains/kotlin/maven/kapt/KaptJVMCompilerMojo.java
@@ -303,6 +303,7 @@ public class KaptJVMCompilerMojo extends K2JVMCompileMojo {
                 oos.writeUTF(entry.getValue());
             }
 
+            oos.flush();
             return Base64.getEncoder().encodeToString(os.toByteArray());
         } catch (IOException e) {
             // Should not occur