diff --git a/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java b/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java
index 0bd1792f8..47ba879fd 100644
--- a/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java
+++ b/spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/reactor/ReactorSleuth.java
@@ -102,7 +102,7 @@ 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-core/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java
index 9c78dc1f5..8cf19b972 100644
--- a/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java
+++ b/spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/reactor/SpanSubscriberTests.java
@@ -225,6 +225,18 @@ public class SpanSubscriberTests {
 		then(spanInOperation).hasValue(initSpan.context().traceId()); // Expecting <AtomicReference[null]> to have value: <1L> but did not.
 	}
 
+	// #646
+	@Test
+	public void should_work_for_mono_just_with_flat_map() {
+		Span initSpan = this.tracer.nextSpan().name("foo").start();
+		log.info("Hello");
+
+		try (Tracer.SpanInScope ws = this.tracer.withSpanInScope(initSpan)) {
+			Mono.just("value1").flatMap(request -> Mono.just("value2").then(Mono.just("foo")))
+					.map(a -> "qwe").block();
+		}
+	}
+
 	@AfterClass
 	public static void cleanup() {
 		Hooks.resetOnLastOperator();