diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala b/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala
index 2cf2c043a3..4afede1fb2 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala
@@ -363,29 +363,31 @@ class FlowGraphBuilder private (graph: Graph[FlowGraphInternal.Vertex, LkDiEdge]
     require(graph.find(node) == None, s"[$node] instance is already used in this flow graph")
 
   private def checkJunctionInPortPrecondition(junction: JunctionInPort[_]): Unit = {
-    // FIXME: Reenable checks
-    //    junction match {
-    //      case _: FanOutOperation[_] if in ⇒
-    //        graph.find(junction.vertex) match {
-    //          case Some(existing) if existing.incoming.nonEmpty ⇒
-    //            throw new IllegalArgumentException(s"Fan-out [$junction] is already attached to input [${existing.incoming.head}]")
-    //          case _ ⇒ // ok
-    //        }
-    //      case _ ⇒ // ok
-    //    }
+    junction.vertex match {
+      case iv: InternalVertex ⇒
+        graph.find(iv) match {
+          case Some(node) ⇒
+            require(
+              (node.inDegree + 1) <= iv.maximumInputCount,
+              s"${node.value}) must have at most ${iv.maximumInputCount} incoming edges")
+          case _ ⇒ // ok
+        }
+      case _ ⇒ // ok, no checks here
+    }
   }
 
   private def checkJunctionOutPortPrecondition(junction: JunctionOutPort[_]): Unit = {
-    // FIXME: Reenable checks
-    //    junction match {
-    //      case _: FanInOperation[_] if !in ⇒
-    //        graph.find(junction.vertex) match {
-    //          case Some(existing) if existing.outgoing.nonEmpty ⇒
-    //            throw new IllegalArgumentException(s"Fan-in [$junction] is already attached to output [${existing.outgoing.head}]")
-    //          case _ ⇒ // ok
-    //        }
-    //      case _ ⇒ // ok
-    //    }
+    junction.vertex match {
+      case iv: InternalVertex ⇒
+        graph.find(iv) match {
+          case Some(node) ⇒
+            require(
+              (node.outDegree + 1) <= iv.maximumOutputCount,
+              s"${node.value}) must have at most ${iv.maximumOutputCount} outgoing edges")
+          case _ ⇒ // ok
+        }
+      case _ ⇒ // ok, no checks here
+    }
   }
 
   /**
@@ -431,22 +433,21 @@ class FlowGraphBuilder private (graph: Graph[FlowGraphInternal.Vertex, LkDiEdge]
       throw new IllegalArgumentException("Undefined sources or sinks: " + formatted.mkString(", "))
     }
 
-    // we will be able to relax these checks
     graph.nodes.foreach { node ⇒
       node.value match {
         case v: InternalVertex ⇒
           require(
             node.inDegree >= v.minimumInputCount,
-            s"${node.incoming}) must have at least ${v.minimumInputCount} incoming edges")
+            s"$v must have at least ${v.minimumInputCount} incoming edges")
           require(
             node.inDegree <= v.maximumInputCount,
-            s"${node.incoming}) must have at most ${v.maximumInputCount} incoming edges")
+            s"$v must have at most ${v.maximumInputCount} incoming edges")
           require(
             node.outDegree >= v.minimumOutputCount,
-            s"${node.outgoing.size}) must have at least ${v.minimumOutputCount} outgoing edges")
+            s"$v must have at least ${v.minimumOutputCount} outgoing edges")
           require(
             node.outDegree <= v.maximumOutputCount,
-            s"${node.incoming}) must have at most ${v.maximumOutputCount} outgoing edges")
+            s"$v must have at most ${v.maximumOutputCount} outgoing edges")
         case _ ⇒ // no check for other node types
       }
     }
diff --git a/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala b/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala
index 9dce1e0124..8185f47e6d 100644
--- a/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala
+++ b/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala
@@ -251,5 +251,27 @@ class FlowGraphCompileSpec extends AkkaSpec {
       }.getMessage should include("empty")
     }
 
+    "check maximumInputCount" in {
+      intercept[IllegalArgumentException] {
+        FlowGraph { implicit b ⇒
+          val bcast = Broadcast[String]
+          import FlowGraphImplicits._
+          in1 ~> bcast ~> out1
+          in2 ~> bcast // wrong
+        }
+      }.getMessage should include("at most 1 incoming")
+    }
+
+    "check maximumOutputCount" in {
+      intercept[IllegalArgumentException] {
+        FlowGraph { implicit b ⇒
+          val merge = Merge[String]
+          import FlowGraphImplicits._
+          in1 ~> merge ~> out1
+          merge ~> out2 // wrong
+        }
+      }.getMessage should include("at most 1 outgoing")
+    }
+
   }
 }