diff --git a/extensions/resteasy-reactive/quarkus-resteasy-reactive-kotlin-serialization/runtime/src/main/kotlin/io/quarkus/kotlin/serialization/KotlinSerializationMessageBodyWriter.kt b/extensions/resteasy-reactive/quarkus-resteasy-reactive-kotlin-serialization/runtime/src/main/kotlin/io/quarkus/kotlin/serialization/KotlinSerializationMessageBodyWriter.kt
index 134e5592e7ad6..866c528c7a4e3 100644
--- a/extensions/resteasy-reactive/quarkus-resteasy-reactive-kotlin-serialization/runtime/src/main/kotlin/io/quarkus/kotlin/serialization/KotlinSerializationMessageBodyWriter.kt
+++ b/extensions/resteasy-reactive/quarkus-resteasy-reactive-kotlin-serialization/runtime/src/main/kotlin/io/quarkus/kotlin/serialization/KotlinSerializationMessageBodyWriter.kt
@@ -30,7 +30,7 @@ class KotlinSerializationMessageBodyWriter(private val json: Json) : AllWriteabl
         if (o is String) { // YUK: done in order to avoid adding extra quotes...
             entityStream.write(o.toByteArray(StandardCharsets.UTF_8))
         } else {
-            json.encodeToStream(serializer(o.javaClass), o, entityStream)
+            json.encodeToStream(serializer(genericType), o, entityStream)
         }
     }
 
diff --git a/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/util/types/Types.java b/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/util/types/Types.java
index c347a46697d44..de58afdeb503d 100644
--- a/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/util/types/Types.java
+++ b/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/util/types/Types.java
@@ -180,16 +180,20 @@ public static Type getEffectiveReturnType(Type returnType) {
         if (returnType instanceof ParameterizedType) {
             ParameterizedType type = (ParameterizedType) returnType;
             Type firstTypeArgument = type.getActualTypeArguments()[0];
-            if (type.getRawType() == CompletionStage.class) {
+            Type rawType = type.getRawType();
+            if (rawType == CompletionStage.class) {
                 return getEffectiveReturnType(firstTypeArgument);
             }
-            if (type.getRawType() == Uni.class) {
+            if (rawType == Uni.class) {
                 return getEffectiveReturnType(firstTypeArgument);
             }
-            if (type.getRawType() == Multi.class) {
+            if (rawType == Multi.class) {
                 return getEffectiveReturnType(firstTypeArgument);
             }
-            if (type.getRawType() == RestResponse.class) {
+            if (rawType == RestResponse.class) {
+                return getEffectiveReturnType(firstTypeArgument);
+            }
+            if ("kotlinx.coroutines.flow.Flow".equals(rawType.getTypeName())) { // TODO: this is very ugly and we should probably use some an SPI in order to decouple
                 return getEffectiveReturnType(firstTypeArgument);
             }
             return returnType;