diff --git a/src/common/type/utils.ts b/src/common/type/utils.ts
index be030478..21d91d5a 100644
--- a/src/common/type/utils.ts
+++ b/src/common/type/utils.ts
@@ -122,10 +122,11 @@ export function collectionElementType(type: Readonly<Type>): Type | undefined {
   return undefined;
 }
 
-export function isValidType(t: any): t is Readonly<Type> {
-  if (typeof t === 'string')
-    return PRIMITIVE_TYPES.includes(t as PrimitiveType);
-  if (typeof t !== 'object') return false;
+export function isValidType(t: unknown): t is Readonly<Type> {
+  const isObject = typeof t === 'object' && t !== null;
+  if (typeof t === 'string' || (isObject && 'type' in t))
+    return PRIMITIVE_TYPES.includes((isObject ? t.type : t) as PrimitiveType);
+  if (!isObject) return false;
   if (!('kind' in t)) return false;
   return (
     t.kind === 'signature' ||
diff --git a/src/compute-engine/boxed-expression/utils.ts b/src/compute-engine/boxed-expression/utils.ts
index 30410f84..55c90bf0 100644
--- a/src/compute-engine/boxed-expression/utils.ts
+++ b/src/compute-engine/boxed-expression/utils.ts
@@ -12,6 +12,8 @@ import { DEFINITIONS_INEQUALITIES } from '../latex-syntax/dictionary/definitions
 
 import { MACHINE_PRECISION } from '../numerics/numeric';
 import { Type } from '../../common/type/types';
+import { isValidType } from '../../common/type/utils';
+import { isSubtype } from '../../common/type/subtype';
 
 export function isBoxedExpression(x: unknown): x is BoxedExpression {
   return typeof x === 'object' && x !== null && 'engine' in x;
@@ -182,17 +184,26 @@ export function normalizeFlags(
   return result as NumericFlags;
 }
 
-export function isSymbolDefinition(def: any): def is SymbolDefinition {
+export function isSymbolDefinition(def: unknown): def is SymbolDefinition {
   if (def === undefined || def === null || typeof def !== 'object')
     return false;
 
   if (isBoxedExpression(def)) return false;
 
-  if ('value' in def || 'constant' in def || 'inferred' in def) {
-    if ('type' in def && typeof def.type === 'function') {
-      throw new Error(
-        'The `type` field of a symbol definition should be of type `string`'
-      );
+  if (
+    'type' in def ||
+    'value' in def ||
+    'constant' in def ||
+    'inferred' in def
+  ) {
+    if ('type' in def) {
+      if (!isValidType(def.type))
+        throw new Error(`Invalid type: "${def.type}"`);
+      if (isSubtype('function', def.type)) {
+        throw new Error(
+          'The `type` field of a symbol definition should be of type `string`'
+        );
+      }
     }
 
     if ('signature' in def) {
diff --git a/src/compute-engine/compute-engine.ts b/src/compute-engine/compute-engine.ts
index 59dbf148..7d036c71 100755
--- a/src/compute-engine/compute-engine.ts
+++ b/src/compute-engine/compute-engine.ts
@@ -1371,8 +1371,10 @@ export class ComputeEngine implements IComputeEngine {
     if (
       typeof def === 'object' &&
       'type' in def &&
-      ((typeof def.type === 'string' && isSubtype(def.type, 'function')) ||
-        (def.type && isValidType(def.type)))
+      !(
+        (typeof def.type === 'string' && isSubtype(def.type, 'function')) ||
+        (def.type && isValidType(def.type))
+      )
     ) {
       throw new Error(
         `Invalid definition for "${id}": use a FunctionDefinition to define a function or use 'ce.declare("${id}", "function")'`