diff --git a/compiler/gen_wasm/src/backend.rs b/compiler/gen_wasm/src/backend.rs
index aba1aaa29d..b0bbf6a122 100644
--- a/compiler/gen_wasm/src/backend.rs
+++ b/compiler/gen_wasm/src/backend.rs
@@ -805,12 +805,19 @@ impl<'a> WasmBackend<'a> {
                 // Zig's "fast calling convention" packs structs into CPU registers (stack machine slots) if possible.
                 // If they're small enough they can go into an I32 or I64. If they're big, they're pointers (I32).
                 for arg in arguments {
-                    param_types.push(match self.storage.get(arg) {
+                    match self.storage.get(arg) {
                         StoredValue::StackMemory { size, .. } if *size > 4 && *size <= 8 => {
-                            ValueType::I64
+                            param_types.push(ValueType::I64);
                         }
-                        stored => stored.value_type(),
-                    });
+                        StoredValue::StackMemory { size, .. } if *size == 16 => {
+                            // the exception to the rule: A Decimal is passed as two i64s
+                            param_types.push(ValueType::I64);
+                            param_types.push(ValueType::I64);
+                        }
+                        stored => {
+                            param_types.push(stored.value_type());
+                        }
+                    }
                 }
 
                 let signature_index = self.module.types.insert(Signature {
diff --git a/compiler/gen_wasm/src/low_level.rs b/compiler/gen_wasm/src/low_level.rs
index d94c6f8c7c..da096ec47b 100644
--- a/compiler/gen_wasm/src/low_level.rs
+++ b/compiler/gen_wasm/src/low_level.rs
@@ -63,12 +63,16 @@ pub fn decode_low_level<'a>(
             return NotImplemented;
         }
 
-        NumAdd => match ret_layout.value_type() {
-            I32 => code_builder.i32_add(),
-            I64 => code_builder.i64_add(),
-            F32 => code_builder.f32_add(),
-            F64 => code_builder.f64_add(),
-        },
+        NumAdd => {
+            return BuiltinCall(bitcode::DEC_ADD_WITH_OVERFLOW);
+
+            match ret_layout.value_type() {
+                I32 => code_builder.i32_add(),
+                I64 => code_builder.i64_add(),
+                F32 => code_builder.f32_add(),
+                F64 => code_builder.f64_add(),
+            }
+        }
         NumAddWrap => match ret_layout.value_type() {
             I32 => {
                 code_builder.i32_add();
diff --git a/compiler/test_gen/src/gen_num.rs b/compiler/test_gen/src/gen_num.rs
index 0adbf3e867..ecf899f1b4 100644
--- a/compiler/test_gen/src/gen_num.rs
+++ b/compiler/test_gen/src/gen_num.rs
@@ -609,7 +609,7 @@ fn gen_float_eq() {
 }
 
 #[test]
-#[cfg(any(feature = "gen-llvm"))]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
 fn gen_add_dec() {
     assert_evals_to!(
         indoc!(