diff --git a/src/agents/impl/instance-agents-impl.ts b/src/agents/impl/instance-agents-impl.ts
index 54a9e17..c30e185 100644
--- a/src/agents/impl/instance-agents-impl.ts
+++ b/src/agents/impl/instance-agents-impl.ts
@@ -362,6 +362,10 @@ class InstanceAgentsImpl implements InstanceAgentsInternal {
         if (protoId !== undefined) {
             if (am === undefined || am.meta.protoId !== undefined) {
                 return protoId;
+            } else {
+                throw new RegalError(
+                    "Failed registering the agent's prototype because a bad protoId was already set"
+                );
             }
         } else {
             protoId = StaticPrototypeRegistry.getPrototypeIdOrDefault(agent);
diff --git a/test/unit/agents.test.ts b/test/unit/agents.test.ts
index eb1962c..7ad5787 100644
--- a/test/unit/agents.test.ts
+++ b/test/unit/agents.test.ts
@@ -1993,6 +1993,16 @@ describe("Agents", function() {
                     RegalError
                 );
             });
+
+            it("Throws a RegalError if an agent's protoId is forced to some value", function() {
+                Game.init(MD);
+                const myGame = buildGameInstance();
+
+                const _d = new Dummy("D1", 1);
+                _d.meta.protoId = "foo" as any;
+
+                expect(() => myGame.using(_d)).to.throw(RegalError);
+            });
         });
     });