diff --git a/aws-runtime/http-client-engine-crt/common/src/aws/sdk/kotlin/runtime/http/engine/crt/RequestUtil.kt b/aws-runtime/http-client-engine-crt/common/src/aws/sdk/kotlin/runtime/http/engine/crt/RequestUtil.kt
index 3b71bbbce5f..060a7f7aba6 100644
--- a/aws-runtime/http-client-engine-crt/common/src/aws/sdk/kotlin/runtime/http/engine/crt/RequestUtil.kt
+++ b/aws-runtime/http-client-engine-crt/common/src/aws/sdk/kotlin/runtime/http/engine/crt/RequestUtil.kt
@@ -42,7 +42,11 @@ internal fun HttpRequest.toCrtRequest(callContext: CoroutineContext): aws.sdk.ko
         headers.forEach { key, values -> appendAll(key, values) }
     }
 
-    val contentLength = body.contentLength?.toString() ?: headers[CONTENT_LENGTH_HEADER]
+    val bodyLen = body.contentLength
+    val contentLength = when {
+        bodyLen != null -> if (bodyLen > 0) bodyLen.toString() else null
+        else -> headers[CONTENT_LENGTH_HEADER]
+    }
     contentLength?.let { crtHeaders.append(CONTENT_LENGTH_HEADER, it) }
 
     return aws.sdk.kotlin.crt.http.HttpRequest(method.name, url.encodedPath, crtHeaders.build(), bodyStream)
diff --git a/aws-runtime/http-client-engine-crt/common/test/aws/sdk/kotlin/runtime/http/engine/crt/RequestConversionTest.kt b/aws-runtime/http-client-engine-crt/common/test/aws/sdk/kotlin/runtime/http/engine/crt/RequestConversionTest.kt
index 5afdb19c389..8f352fcc3a2 100644
--- a/aws-runtime/http-client-engine-crt/common/test/aws/sdk/kotlin/runtime/http/engine/crt/RequestConversionTest.kt
+++ b/aws-runtime/http-client-engine-crt/common/test/aws/sdk/kotlin/runtime/http/engine/crt/RequestConversionTest.kt
@@ -91,4 +91,18 @@ class RequestConversionTest {
         val crtBody = crtRequest.body as ReadChannelBodyStream
         crtBody.cancel()
     }
+
+    @Test
+    fun testEngineDoesNotAddContentLengthHeaderForEmptyBody() {
+        val request = HttpRequest(
+            HttpMethod.POST,
+            Url.parse("https://test.aws.com?foo=bar"),
+            Headers.Empty,
+            HttpBody.Empty
+        )
+
+        val testContext = EmptyCoroutineContext + Job()
+        val crtRequest = request.toCrtRequest(testContext)
+        assertFalse(crtRequest.headers.contains("Content-Length"))
+    }
 }
diff --git a/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/RestJson1.kt b/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/RestJson1.kt
index 9aaffe1ba16..08fca8aa548 100644
--- a/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/RestJson1.kt
+++ b/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/RestJson1.kt
@@ -7,10 +7,17 @@ package aws.sdk.kotlin.codegen.protocols
 import aws.sdk.kotlin.codegen.protocols.core.AwsHttpBindingProtocolGenerator
 import aws.sdk.kotlin.codegen.protocols.json.JsonHttpBindingProtocolGenerator
 import software.amazon.smithy.aws.traits.protocols.RestJson1Trait
+import software.amazon.smithy.kotlin.codegen.core.KotlinWriter
+import software.amazon.smithy.kotlin.codegen.core.RuntimeTypes
+import software.amazon.smithy.kotlin.codegen.core.defaultName
+import software.amazon.smithy.kotlin.codegen.core.withBlock
 import software.amazon.smithy.kotlin.codegen.rendering.protocol.*
 import software.amazon.smithy.model.Model
+import software.amazon.smithy.model.knowledge.HttpBinding
+import software.amazon.smithy.model.shapes.OperationShape
 import software.amazon.smithy.model.shapes.ServiceShape
 import software.amazon.smithy.model.shapes.ShapeId
+import software.amazon.smithy.model.shapes.StructureShape
 
 /**
  * Handles generating the aws.protocols#restJson1 protocol for services.
@@ -24,4 +31,32 @@ class RestJson1 : JsonHttpBindingProtocolGenerator() {
 
     override fun getProtocolHttpBindingResolver(model: Model, serviceShape: ServiceShape): HttpBindingResolver =
         HttpTraitResolver(model, serviceShape, "application/json")
+
+    override fun renderSerializeHttpBody(
+        ctx: ProtocolGenerator.GenerationContext,
+        op: OperationShape,
+        writer: KotlinWriter
+    ) {
+        super.renderSerializeHttpBody(ctx, op, writer)
+
+        val resolver = getProtocolHttpBindingResolver(ctx.model, ctx.service)
+        if (!resolver.hasHttpBody(op)) return
+
+        // restjson1 has some different semantics and expectations around empty structures bound via @httpPayload trait
+        //   * empty structures get serialized to `{}`
+        // see: https://github.com/awslabs/smithy/pull/924
+        val requestBindings = resolver.requestBindings(op)
+        val httpPayload = requestBindings.firstOrNull { it.location == HttpBinding.Location.PAYLOAD }
+        if (httpPayload != null) {
+            // explicit payload member as the sole payload
+            val memberName = httpPayload.member.defaultName()
+            val target = ctx.model.expectShape(httpPayload.member.target)
+            if (target is StructureShape) {
+                writer.withBlock("if (input.#L == null) {", "}", memberName) {
+                    addImport(RuntimeTypes.Http.ByteArrayContent)
+                    write("builder.body = #T(#S.encodeToByteArray())", RuntimeTypes.Http.ByteArrayContent, "{}")
+                }
+            }
+        }
+    }
 }
diff --git a/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/core/AwsHttpBindingProtocolGenerator.kt b/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/core/AwsHttpBindingProtocolGenerator.kt
index cb6d09c615f..f7a2ad34f12 100644
--- a/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/core/AwsHttpBindingProtocolGenerator.kt
+++ b/codegen/smithy-aws-kotlin-codegen/src/main/kotlin/aws/sdk/kotlin/codegen/protocols/core/AwsHttpBindingProtocolGenerator.kt
@@ -78,13 +78,6 @@ abstract class AwsHttpBindingProtocolGenerator : HttpBindingProtocolGenerator()
                 // FIXME - document type not fully supported yet, see https://github.com/awslabs/smithy-kotlin/issues/123
                 "PutAndGetInlineDocumentsInput",
 
-                // aws-sdk-kotlin#390
-                "RestJsonHttpWithHeaderMemberNoModeledBody",
-                "RestJsonHttpWithNoModeledBody",
-                "RestJsonHttpWithEmptyBlobPayload",
-                "RestJsonHttpWithEmptyStructurePayload",
-                "RestJsonHttpWithHeadersButNoPayload",
-
                 // smithy-kotlin#519
                 "SimpleScalarPropertiesWithWhiteSpace",
                 "SimpleScalarPropertiesPureWhiteSpace",
diff --git a/gradle.properties b/gradle.properties
index 93451222220..5dc3a656508 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -10,7 +10,7 @@ org.gradle.jvmargs=-Xmx6g -XX:MaxPermSize=6g -XX:MaxMetaspaceSize=1G
 sdkVersion=0.10.0-SNAPSHOT
 
 # codegen
-smithyVersion=1.13.0
+smithyVersion=1.13.1
 smithyGradleVersion=0.5.3
 # smithy-kotlin codegen and runtime are versioned together
 smithyKotlinVersion=0.7.1-SNAPSHOT