diff --git a/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/integration/middlewares/handlers/HttpBodyMiddleware.kt b/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/integration/middlewares/handlers/HttpBodyMiddleware.kt
index 70ce9d0e9..76e1b467f 100644
--- a/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/integration/middlewares/handlers/HttpBodyMiddleware.kt
+++ b/smithy-swift-codegen/src/main/kotlin/software/amazon/smithy/swift/codegen/integration/middlewares/handlers/HttpBodyMiddleware.kt
@@ -102,7 +102,32 @@ class HttpBodyMiddleware(
                     renderEncodedBodyAddedToRequest(bodyDeclaration, dataDeclaration)
                 }
             }
-            ShapeType.DOCUMENT, ShapeType.STRUCTURE, ShapeType.UNION -> {
+            ShapeType.STRUCTURE, ShapeType.UNION -> {
+                // delegate to the member encode function
+                writer.openBlock("do {", "} catch let err {") {
+                    writer.write("let encoder = context.getEncoder()")
+                    writer.openBlock("if let $memberName = input.operationInput.$memberName {", "} else {") {
+                        writer.write("let $dataDeclaration = try encoder.encode(\$L)", memberName)
+                        renderEncodedBodyAddedToRequest(bodyDeclaration, dataDeclaration)
+                    }
+                    writer.indent()
+                    writer.openBlock("if encoder is JSONEncoder {", "} else if encoder is XMLEncoder {") {
+                        writer.write("// Encode an empty body as an empty structure in JSON")
+                        writer.write("let \$L = \"{}\".data(using: .utf8)!", dataDeclaration)
+                        renderEncodedBodyAddedToRequest(bodyDeclaration, dataDeclaration)
+                    }
+                    writer.indent()
+                    writer.write("// Encode an empty body as an empty string in XML")
+                    writer.write("let \$L = \"\".data(using: .utf8)!", dataDeclaration)
+                    renderEncodedBodyAddedToRequest(bodyDeclaration, dataDeclaration)
+                    writer.dedent()
+                    writer.write("}")
+                    writer.dedent()
+                    writer.write("}")
+                }
+                renderErrorCase()
+            }
+            ShapeType.DOCUMENT -> {
                 writer.openBlock("do {", "} catch let err {") {
                     writer.openBlock("if let $memberName = input.operationInput.$memberName {", "}") {
                         writer.write("let encoder = context.getEncoder()")
diff --git a/smithy-swift-codegen/src/test/kotlin/HttpBodyMiddlewareTests.kt b/smithy-swift-codegen/src/test/kotlin/HttpBodyMiddlewareTests.kt
index bc1e82114..1ed3db63c 100644
--- a/smithy-swift-codegen/src/test/kotlin/HttpBodyMiddlewareTests.kt
+++ b/smithy-swift-codegen/src/test/kotlin/HttpBodyMiddlewareTests.kt
@@ -149,11 +149,23 @@ class HttpBodyMiddlewareTests {
                 Self.Context == H.Context
                 {
                     do {
+                        let encoder = context.getEncoder()
                         if let payload1 = input.operationInput.payload1 {
-                            let encoder = context.getEncoder()
                             let payload1data = try encoder.encode(payload1)
                             let payload1body = ClientRuntime.HttpBody.data(payload1data)
                             input.builder.withBody(payload1body)
+                        } else {
+                            if encoder is JSONEncoder {
+                                // Encode an empty body as an empty structure in JSON
+                                let payload1data = "{}".data(using: .utf8)!
+                                let payload1body = ClientRuntime.HttpBody.data(payload1data)
+                                input.builder.withBody(payload1body)
+                            } else if encoder is XMLEncoder {
+                                // Encode an empty body as an empty string in XML
+                                let payload1data = "".data(using: .utf8)!
+                                let payload1body = ClientRuntime.HttpBody.data(payload1data)
+                                input.builder.withBody(payload1body)
+                            }
                         }
                     } catch let err {
                         throw SdkError<ExplicitStructOutputError>.client(ClientRuntime.ClientError.serializationFailed(err.localizedDescription))