diff --git a/src/behavior.ts b/src/behavior.ts
index c566388..82674af 100644
--- a/src/behavior.ts
+++ b/src/behavior.ts
@@ -139,7 +139,7 @@ function refresh<A>(b: Behavior<A>, t: number) {
 
 export function isBehavior<A>(b: unknown): b is Behavior<A> {
   return (
-    (typeof b === "object" && "at" in b && !isPlaceholder(b)) ||
+    (typeof b === "object" && "at" in b && "pull" in b && !isPlaceholder(b)) ||
     (isPlaceholder(b) && (b.source === undefined || isBehavior(b.source)))
   );
 }
diff --git a/src/common.ts b/src/common.ts
index 072f087..4a98753 100644
--- a/src/common.ts
+++ b/src/common.ts
@@ -5,7 +5,7 @@ import { tick } from "./clock";
 export type Time = number;
 
 function isBehavior(b: unknown): b is Behavior<unknown> {
-  return typeof b === "object" && "at" in b;
+  return typeof b === "object" && "at" in b && "pull" in b;
 }
 
 export type PullHandler = (pull: (t?: number) => void) => () => void;