diff --git a/src/value.ts b/src/value.ts
index af1930e..be429e3 100644
--- a/src/value.ts
+++ b/src/value.ts
@@ -61,7 +61,7 @@ export function toValue(v: any): Value /* throws */ {
     case 'string':
       return { type: 'string', value: v }
     case 'function':
-      throw "can't toValue() a js function yet"
+      throw new Error("can't toValue() a js function yet")
     case 'object':
       const dict: Dict = new Map()
 
@@ -69,7 +69,7 @@ export function toValue(v: any): Value /* throws */ {
 
       return { type: 'dict', value: dict }
     default:
-      throw `can't toValue this: ${v}`
+      throw new Error(`can't toValue this: ${v}`)
   }
 }
 
diff --git a/src/vm.ts b/src/vm.ts
index 30ee039..7b8cd24 100644
--- a/src/vm.ts
+++ b/src/vm.ts
@@ -37,7 +37,8 @@ export class VM {
     const value = this.scope.get(name)
 
     if (!value) throw new Error(`Can't find ${name}`)
-    if (value.type !== 'function' && value.type !== 'native') throw new Error(`Can't call ${name}`)
+    if (value.type !== 'function' && value.type !== 'native')
+      throw new Error(`Can't call ${name}`)
 
     if (value.type === 'native') {
       return await this.callNative(value.fn, args)
@@ -725,7 +726,7 @@ export class VM {
         break
 
       default:
-        throw `Unknown op: ${instruction.op}`
+        throw new Error(`Unknown op: ${instruction.op}`)
     }
   }