diff --git a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt
index 17319765ff..69b9b84e0d 100644
--- a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt
+++ b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt
@@ -272,6 +272,8 @@ class HttpBoundProtocolPayloadGenerator(
                                 """,
                             )
                             is StructureShape -> rust("#T()", serializerGenerator.unsetStructure(targetShape))
+                            is UnionShape -> throw CodegenException("Currently unsupported. Tracking issue: https://github.com/awslabs/smithy-rs/issues/1896")
+                            else -> throw CodegenException("`httpPayload` on member shapes targeting shapes of type ${targetShape.type} is unsupported")
                         }
                     }
                 }