diff --git a/spring-cloud-sleuth-reactor/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java b/spring-cloud-sleuth-reactor/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java
index d8c6b244a..50efedea5 100644
--- a/spring-cloud-sleuth-reactor/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java
+++ b/spring-cloud-sleuth-reactor/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java
@@ -45,7 +45,7 @@ public abstract class ReactorSleuth {
 	}
 
 	private static final Predicate<Scannable> POINTCUT_FILTER =
-			s ->  !(s instanceof Fuseable.ScalarCallable);
+			s ->  !(s instanceof Fuseable.ScalarCallable) && s.isScanAvailable();
 
 	private ReactorSleuth() {
 	}
diff --git a/spring-cloud-sleuth-reactor/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java b/spring-cloud-sleuth-reactor/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java
index d1ff48b5c..9cb7ff326 100644
--- a/spring-cloud-sleuth-reactor/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java
+++ b/spring-cloud-sleuth-reactor/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java
@@ -179,6 +179,27 @@ public class SpanSubscriberTests {
 		tracer.close(foo2);
 	}
 
+	// #646
+	@Test
+	public void should_work_for_mono_just_with_flat_map() {
+
+		Span span = this.tracer.createSpan("foo");
+		log.info("Hello");
+
+		Mono.just("value1")
+				.flatMap(request ->
+						Mono.just("value2")
+								.then(Mono.just("foo"))
+				)
+				.map(a -> "qwe")
+			.block();
+
+		Awaitility.await().untilAsserted(() -> {
+			then(ExceptionUtils.getLastException()).isNull();
+		});
+		then(this.tracer.getCurrentSpan()).isNull();
+	}
+
 	@AfterClass
 	public static void cleanup() {
 		Hooks.resetOnLastOperator();