diff --git a/src/main/kotlin/dev/silenium/libs/flows/impl/FlowGraphImpl.kt b/src/main/kotlin/dev/silenium/libs/flows/impl/FlowGraphImpl.kt
index 236bf11..0a7926d 100644
--- a/src/main/kotlin/dev/silenium/libs/flows/impl/FlowGraphImpl.kt
+++ b/src/main/kotlin/dev/silenium/libs/flows/impl/FlowGraphImpl.kt
@@ -3,6 +3,9 @@ package dev.silenium.libs.flows.impl
 import dev.silenium.libs.flows.api.*
 import kotlinx.coroutines.*
 import kotlinx.coroutines.flow.map
+import kotlin.contracts.ExperimentalContracts
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
 import kotlin.coroutines.CoroutineContext
 import kotlin.reflect.KClass
 
@@ -147,10 +150,14 @@ internal fun FlowGraph.builder() = FlowGraphConfigScopeImpl(this)
  * @see FlowGraph
  * @see CoroutineContext
  */
+@OptIn(ExperimentalContracts::class)
 suspend fun FlowGraph(
     coroutineContext: CoroutineContext = Dispatchers.Default,
     block: FlowGraphConfigScope.() -> Unit,
-): FlowGraph = FlowGraphImpl(coroutineContext).builder().apply(block).configure().getOrThrow()
+): FlowGraph {
+    contract { callsInPlace(block, InvocationKind.EXACTLY_ONCE) }
+    return FlowGraphImpl(coroutineContext).builder().apply(block).configure().getOrThrow()
+}
 
 /**
  * Creates a new [FlowGraph] with the given [coroutineScope] and [block] configuration.
@@ -163,7 +170,11 @@ suspend fun FlowGraph(
  * @see FlowGraph
  * @see CoroutineScope
  */
+@OptIn(ExperimentalContracts::class)
 suspend fun FlowGraph(
     coroutineScope: CoroutineScope,
     block: FlowGraphConfigScope.() -> Unit,
-): FlowGraph = FlowGraphImpl(coroutineScope).builder().apply(block).configure().getOrThrow()
+): FlowGraph {
+    contract { callsInPlace(block, InvocationKind.EXACTLY_ONCE) }
+    return FlowGraphImpl(coroutineScope).builder().apply(block).configure().getOrThrow()
+}
diff --git a/src/test/kotlin/dev/silenium/libs/flows/impl/FlowGraphImplTest.kt b/src/test/kotlin/dev/silenium/libs/flows/impl/FlowGraphImplTest.kt
index f981723..02c0621 100644
--- a/src/test/kotlin/dev/silenium/libs/flows/impl/FlowGraphImplTest.kt
+++ b/src/test/kotlin/dev/silenium/libs/flows/impl/FlowGraphImplTest.kt
@@ -1,5 +1,6 @@
 package dev.silenium.libs.flows.impl
 
+import dev.silenium.libs.flows.api.SourceFlowGraphElement
 import dev.silenium.libs.flows.buffer.BufferSink
 import dev.silenium.libs.flows.buffer.BufferSource
 import dev.silenium.libs.flows.test.Base64Buffer
@@ -16,8 +17,9 @@ import kotlinx.coroutines.flow.firstOrNull
 
 class FlowGraphImplTest : FunSpec({
     test("FlowGraphBuilder") {
+        val source: SourceFlowGraphElement<Base64Buffer, DataType, BufferSource<Base64Buffer, DataType>>
         val graph = FlowGraph(CoroutineScope(Dispatchers.Default)) {
-            val source = source(BufferSource<Base64Buffer, DataType>(0u to DataType.BASE64), "buffer-source")
+            source = source(BufferSource(0u to DataType.BASE64), "buffer-source")
             val sink = sink(BufferSink<ByteArray, DataType>(), "buffer-sink")
             val decoder = transformer(Base64Decoder(), "base64-decoder")
             connect(source to decoder)
@@ -25,7 +27,6 @@ class FlowGraphImplTest : FunSpec({
                 sourcePad + 1u
             }
         }
-        val source = graph.source<BufferSource<Base64Buffer, DataType>>("buffer-source")!!
         val sink = graph.sink<BufferSink<ByteArray, DataType>>("buffer-sink")!!
 
         val input = "test"