diff --git a/cranelift/codegen/src/isa/s390x/abi.rs b/cranelift/codegen/src/isa/s390x/abi.rs
index c66d3ac2a0..9d27be3012 100644
--- a/cranelift/codegen/src/isa/s390x/abi.rs
+++ b/cranelift/codegen/src/isa/s390x/abi.rs
@@ -275,7 +275,6 @@ impl ABIMachineSpec for S390xMachineDeps {
             } else if call_conv.extends_wasmtime() {
                 panic!("i128 args/return values not supported in the Wasmtime ABI");
             } else {
-                assert!(param.extension == ir::ArgumentExtension::None);
                 // We must pass this by implicit reference.
                 if args_or_rets == ArgsOrRets::Rets {
                     // For return values, just force them to memory.
diff --git a/cranelift/codegen/src/isa/s390x/inst.isle b/cranelift/codegen/src/isa/s390x/inst.isle
index 458dfe4789..815513f046 100644
--- a/cranelift/codegen/src/isa/s390x/inst.isle
+++ b/cranelift/codegen/src/isa/s390x/inst.isle
@@ -2975,9 +2975,9 @@
 
 ;; Helper to compute the type of an implicitly extended argument/return value.
 (decl abi_ext_ty (ArgumentExtension Type) Type)
-(rule (abi_ext_ty (ArgumentExtension.None) ty) ty)
-(rule (abi_ext_ty (ArgumentExtension.Uext) _) $I64)
-(rule (abi_ext_ty (ArgumentExtension.Sext) _) $I64)
+(rule 0 (abi_ext_ty _ ty) ty)
+(rule 1 (abi_ext_ty (ArgumentExtension.Uext) (gpr32_ty _)) $I64)
+(rule 1 (abi_ext_ty (ArgumentExtension.Sext) (gpr32_ty _)) $I64)
 
 
 ;; Helpers for generating immediate values ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
diff --git a/cranelift/fuzzgen/src/function_generator.rs b/cranelift/fuzzgen/src/function_generator.rs
index 2383fbdb54..f98212c9c3 100644
--- a/cranelift/fuzzgen/src/function_generator.rs
+++ b/cranelift/fuzzgen/src/function_generator.rs
@@ -502,6 +502,46 @@ fn valid_for_target(triple: &Triple, op: Opcode, args: &[Type], rets: &[Type]) -
             )
         }
 
+        Architecture::S390x => {
+            exceptions!(
+                (Opcode::IaddCout),
+                (Opcode::Udiv, &[I128, I128]),
+                (Opcode::Sdiv, &[I128, I128]),
+                (Opcode::Urem, &[I128, I128]),
+                (Opcode::Srem, &[I128, I128]),
+                (Opcode::Smin),
+                (Opcode::Smax),
+                (Opcode::Umin),
+                (Opcode::Umax),
+                (Opcode::Band, &[F32, F32]),
+                (Opcode::Band, &[F64, F64]),
+                (Opcode::Bor, &[F32, F32]),
+                (Opcode::Bor, &[F64, F64]),
+                (Opcode::Bxor, &[F32, F32]),
+                (Opcode::Bxor, &[F64, F64]),
+                (Opcode::Bnot, &[F32, F32]),
+                (Opcode::Bnot, &[F64, F64]),
+                (Opcode::BandNot, &[F32, F32]),
+                (Opcode::BandNot, &[F64, F64]),
+                (Opcode::BorNot, &[F32, F32]),
+                (Opcode::BorNot, &[F64, F64]),
+                (Opcode::BxorNot, &[F32, F32]),
+                (Opcode::BxorNot, &[F64, F64]),
+                (Opcode::FcvtToUint, &[F32], &[I128]),
+                (Opcode::FcvtToUint, &[F64], &[I128]),
+                (Opcode::FcvtToUintSat, &[F32], &[I128]),
+                (Opcode::FcvtToUintSat, &[F64], &[I128]),
+                (Opcode::FcvtToSint, &[F32], &[I128]),
+                (Opcode::FcvtToSint, &[F64], &[I128]),
+                (Opcode::FcvtToSintSat, &[F32], &[I128]),
+                (Opcode::FcvtToSintSat, &[F64], &[I128]),
+                (Opcode::FcvtFromUint, &[I128], &[F32]),
+                (Opcode::FcvtFromUint, &[I128], &[F64]),
+                (Opcode::FcvtFromSint, &[I128], &[F32]),
+                (Opcode::FcvtFromSint, &[I128], &[F64]),
+            )
+        }
+
         Architecture::Riscv64(_) => {
             exceptions!(
                 // TODO
diff --git a/fuzz/Cargo.toml b/fuzz/Cargo.toml
index 8ee4559574..11b6ff3d02 100644
--- a/fuzz/Cargo.toml
+++ b/fuzz/Cargo.toml
@@ -10,7 +10,7 @@ cargo-fuzz = true
 [dependencies]
 anyhow = { workspace = true }
 once_cell = { workspace = true }
-cranelift-codegen = { workspace = true, features = ["incremental-cache", "x86", "arm64", "riscv64"] }
+cranelift-codegen = { workspace = true, features = ["incremental-cache", "x86", "arm64", "s390x", "riscv64"] }
 cranelift-reader = { workspace = true }
 cranelift-wasm = { workspace = true }
 cranelift-filetests = { workspace = true }