diff --git a/SPEC.md b/SPEC.md
index 430314e..4262d6d 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -184,7 +184,8 @@ Performs different operations depending on operand types:
 - If either operand is a string, converts both to strings and concatenates
 - Else if both operands are arrays, concatenates the arrays
 - Else if both operands are dicts, merges them (b's keys overwrite a's keys on conflict)
-- Otherwise, converts both to numbers and performs numeric addition
+- Else if both operands are numbers, performs numeric addition
+- Otherwise, throws an error
 
 **Examples**:
 - `5 + 3` → `8` (numeric addition)
@@ -196,6 +197,12 @@ Performs different operations depending on operand types:
 - `{a: 1} + {b: 2}` → `{a: 1, b: 2}` (dict merge)
 - `{a: 1, b: 2} + {b: 99}` → `{a: 1, b: 99}` (dict merge, b overwrites)
 
+**Invalid operations** (throw errors):
+- `true + false` → Error
+- `null + 5` → Error
+- `[1] + 5` → Error
+- `{a: 1} + 5` → Error
+
 #### SUB
 **Stack**: [a, b] → [a - b]
 
diff --git a/src/vm.ts b/src/vm.ts
index a5f5e76..d55eb8c 100644
--- a/src/vm.ts
+++ b/src/vm.ts
@@ -148,22 +148,20 @@ export class VM {
         const b = this.stack.pop()!
         const a = this.stack.pop()!
 
-        // If either operand is a string, do string concatenation
         if (a.type === 'string' || b.type === 'string') {
           this.stack.push(toValue(toString(a) + toString(b)))
         } else if (a.type === 'array' && b.type === 'array') {
-          // two arrays, concatenate them
           this.stack.push({ type: 'array', value: [...a.value, ...b.value] })
         } else if (a.type === 'dict' && b.type === 'dict') {
-          // two dicts, merge them (b's keys overwrite a's keys)
           const merged = new Map(a.value)
           for (const [key, value] of b.value) {
             merged.set(key, value)
           }
           this.stack.push({ type: 'dict', value: merged })
+        } else if (a.type === 'number' && b.type === 'number') {
+          this.stack.push(toValue(a.value + b.value))
         } else {
-          // Otherwise do numeric addition
-          this.stack.push(toValue(toNumber(a) + toNumber(b)))
+          throw new Error(`ADD: Cannot add ${a.type} and ${b.type}`)
         }
         break
 
diff --git a/tests/functions.test.ts b/tests/functions.test.ts
index fc35e78..5a8bbf4 100644
--- a/tests/functions.test.ts
+++ b/tests/functions.test.ts
@@ -487,7 +487,7 @@ test("TRY_CALL - handles null values", async () => {
 
 test("TRY_CALL - function can access its parameters", async () => {
   const bytecode = toBytecode([
-    ["MAKE_FUNCTION", ["x"], ".body"],
+    ["MAKE_FUNCTION", ["x=0"], ".body"],
     ["STORE", "addFive"],
     ["PUSH", 10],
     ["STORE", "x"],
@@ -501,8 +501,8 @@ test("TRY_CALL - function can access its parameters", async () => {
   ])
 
   const result = await run(bytecode)
-  // Function is called with 0 args, so x inside function should be null
-  // Then we add 5 to null (which coerces to 0)
+  // Function is called with 0 args, so x defaults to 0
+  // Then we add 5 to 0
   expect(result).toEqual({ type: 'number', value: 5 })
 })
 
diff --git a/tests/opcodes.test.ts b/tests/opcodes.test.ts
index 7cc1322..394771f 100644
--- a/tests/opcodes.test.ts
+++ b/tests/opcodes.test.ts
@@ -373,6 +373,87 @@ describe("ADD", () => {
       }
     }
   })
+
+  test("cannot add boolean + boolean", async () => {
+    const bytecode = toBytecode([
+      ["PUSH", true],
+      ["PUSH", false],
+      ["ADD"],
+      ["HALT"]
+    ])
+
+    await expect(run(bytecode)).rejects.toThrow('ADD: Cannot add boolean and boolean')
+  })
+
+  test("cannot add null + number", async () => {
+    const bytecode = toBytecode([
+      ["PUSH", null],
+      ["PUSH", 5],
+      ["ADD"],
+      ["HALT"]
+    ])
+
+    await expect(run(bytecode)).rejects.toThrow('ADD: Cannot add null and number')
+  })
+
+  test("cannot add array + dict", async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 1],
+      ["MAKE_ARRAY", 1],
+      ["MAKE_DICT", 0],
+      ["ADD"],
+      ["HALT"]
+    ])
+
+    await expect(run(bytecode)).rejects.toThrow('ADD: Cannot add array and dict')
+  })
+
+  test("cannot add array + number", async () => {
+    const bytecode = toBytecode([
+      ["PUSH", 1],
+      ["MAKE_ARRAY", 1],
+      ["PUSH", 5],
+      ["ADD"],
+      ["HALT"]
+    ])
+
+    await expect(run(bytecode)).rejects.toThrow('ADD: Cannot add array and number')
+  })
+
+  test("cannot add dict + number", async () => {
+    const bytecode = toBytecode([
+      ["MAKE_DICT", 0],
+      ["PUSH", 5],
+      ["ADD"],
+      ["HALT"]
+    ])
+
+    await expect(run(bytecode)).rejects.toThrow('ADD: Cannot add dict and number')
+  })
+
+  test("cannot add function + number", async () => {
+    const bytecode = toBytecode([
+      ["MAKE_FUNCTION", [], ".body"],
+      ["PUSH", 5],
+      ["ADD"],
+      ["HALT"],
+      [".body:"],
+      ["RETURN"]
+    ])
+
+    await expect(run(bytecode)).rejects.toThrow('ADD: Cannot add function and number')
+  })
+
+  test("cannot add boolean + null", async () => {
+    const bytecode = toBytecode([
+      ["PUSH", true],
+      ["PUSH", null],
+      ["ADD"],
+      ["HALT"]
+    ])
+
+    await expect(run(bytecode)).rejects.toThrow('ADD: Cannot add boolean and null')
+  })
 })
 
 describe("SUB", () => {